light sense

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The consulting staff will bring together distin-guished members of the medical profession in thetwo countries. The visiting staff will be nominatedby the executive medical committee.The governing council of the hospital, which

is likely to have for its patrons the highestpersonages on both sides of the Atlantic, consists ofmany prominent members of the American colonyin London, including Mr. Walter Blackman, Mr.George M. Cassatt, Mr. R. Newton Crane, Mr. WilsonCross, Mr. Clarence Graff, Mr. James Benson

Kennedy, Mr. J. Blair MacAfee, Mr. George A.

Mower, Mr. F. E. Powell, Mr. Henry E. Stoner, Mr.F. C. Van Duzer, and Mr. E. Bradner White, withMr. Philip Franklin as honorary secretary.

LIGHT SENSE.

FOR a study on the " Performance of Night’glasses" we can recommend Bulletin No. 3, byL. C. Martin, D.I.C., &c., published for the Depart-ment of Science and Industrial Research byH.M. Stationery Office. The whole subject of

light sense is indifferently understood. Whenwe pass from bright sunlight to a dimlylighted room we can see nothing until our eyeshave become adapted to the dark. This adapta.tion is supposed to depend principally upon somechange in the relation between the bacillary layerof the retina and the adjoining pigment, and takesthree-quarters of an hour or more to become com-plete, and in the second place upon the dilatationof the pupil, which occurs very rapidly. It is acurious fact that the fovea is physiologicallynight-blind, for a fully dark-adapted or scotopiceye is found to have a central scotoma; round thefovea is a ring that is most sensitive to light, andbeyond this ring the light sense fades graduallytowards the periphery. Again, the scotopic eye iscolour blind, but on the admission of more lightthe first colours recognised are yellow and blue,after them the greens, and, finally, the reds.On a dark night the grass looks grey, whilethe colour of yellow flowers may still be dis-tinguished, but the red geraniums appear black.As there are no rods in the fovea, though they arefound in increasing numbers towards the periphery,it will be seen that there are some good prima faciegrounds for considering the rods as the percipientelements for light sense and the cones for coloursense.

The light sense may be tested in twodifferent ways: (1) Light difference (L.D.): Theestimation of the smallest difference of brightnessthat can be appreciated between two sources of

nearly equal luminosity; and (2) light minimum(L.M.) : the estimation of the smallest quantity oflight that can be recognised in a faintly illuminatedpatch on a black background. Ophthalmologistsfind that L.D. is most affected in diseases of theoptic nerve, but that L.M. is chiefly diminishedin glaucoma and in diseases of the retina andchoroid. It is found that practice increases the L.D.sensibility enormously, and, indeed, the results ofL.D. and L.M. tests vary so greatly in differentindividuals that no definite value can be assignedas the standard of either. When Professor E. M.Barnard (Yerkes Observatory, U.S.A.) found thathe could detect a wire at such a distance thatits diameter only subtended an angle of 0’44"(i.e., less than half a second) at the nodal point ofhis eye, it is a proof of his extraordinary sensibilityto light difference; it is no test of his form sense.

The minimum visible for form sense is 53’1", andis usually taken to be 1’. Many tests thatare supposed to deal with the form sense are

really only testing L.D. Even the dot test cardsthat are so frequently used fall under suspicionfor this reason. In practice this may not bevery serious, as in many callings an adequate lightsensibility is more important than a superior formsense. On a dark night most people have found thatthey see better without their spectacles than withthem, as then they are relying upon theirlight sense and not upon their form sense,which is improved by increased definition. Everyoptical instrument entails some loss of light fromabsorption and from reflection at the surfaces ofthe glass; and yet, as is well known, telescopesenable one to see stars that are invisible tothe naked eye, and night glasses are of in-valuable use to the mariner. The principlesunderlying these two instruments are entirelydifferent.The apparent brightness of a surface in

ordinary circumstances, if the size of the pupilremain constant, simply depends upon the in-trinsic brightness of the surface. A piece ofwhite paper 1 in. square at a distance of 4 ft.is as bright as a piece 1 ft. square. If, however, anobject be so small that its image only falls on partof a bacillary element, unless it be exceedinglybright it may not succeed in stimulating thiselement. When faint stars are observed through atelescope their apparent size is not increased, forthey still subtend an angle less than the minimumvisible, but all the light which falls on the objectglass may by a suitable eye-piece be concentrated onthe observer’s pupil, neglecting the light lost bytransmission through the instrument. The actionof the telescope is tantamount to increasing thearea of the pupil to that of the object glass, and hencethe brightness of the faint star is virtually increased.If, however, the object viewed subtend an angle(e.g., 10’) greater than the minimum visible, nooptical instrument can increase its brightness perunit area. Yet, as is well known, in the dusk anight-glass enables objects to be seen that are

invisible to the naked eye, although telescopes witha magnification of 20 or so will prevent objectsbeing seen that can be dimly distinguished withoutit. Similarly, all microscopists have noted thatwhen high eye-pieces are used the field becomesmuch darker. This is simply because the emergentbeam of light from a high ocular is so thin thatonly part of the pupil is filled with light; indeed,the pupil receives far less light than when a lowereye-piece is used. Now one advantage of a Galileanbinocular is that the section of the emergent beamis never less than that of the pupil and, con-sequently, no light is lost from this cause. But -wehave said that the brightness per unit area

cannot be increased, how then can the visibilityof an object in a dim light be increased bynight-glasses ? All workers agree that theminimum intrinsic brightness necessary for the

perception of an object varies greatly with itssize. It is clear, then, that night-glasses with amagnification of 4 or 6 will increase the visibilityof objects in a dim light owing to their increasedapparent size. We may at once conclude that aslong as the section of the emergent beam is notless than the pupil of the observer’s eye advantagewill be gained by increasing the magnification; butif the sectional area of the emergent beam is lessthan that of the pupil the image may become even

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dimmer than the object when seen by the nakedeye. Piper has pointed out that in dark-adap edeyes the light sensibility is about twice as greatwith both eyes as with one, though in light-adapted eyes no such binocular summation takesplace.

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THE BRITISH SCIENTIFIC PRODUCTS EXHIBITION.

UNDER the auspices of the British Science Guilda very interesting exhibition of British scientificproductions was opened by Lord Crewe at theCentral Hall, Westminster, on Thursday, July 3rd.Encouraged by the success which attended a

similar exhibition held last year at King’s College,the authorities were amply justified in promotinganother exhibition this year, the objects of which areto illustrate recent progress in British science andinvention, and to help the establishment and develop-ment of new British industries. The demonstrationis most instructive, and evidences in many directionsthe capacity and skill of the British manufacturerin acquiring new industries and in restoring oldones on a scientific-basis. There is, indeed, a wealthof products shown, illustrating in the most satis-factory way a determination to press science intoindustrial service and to remove the long-standingreproach of our dependence on other nationsfor the supply of many valuable and importantcommodities. And there is a noble record pre-sented of the work done in the research laboratoriesof the universities and schools of England. All is aneloquent story of the successes gained by the mentrained in the scientific and technical laboratories.Mechanical science, physics, textiles, electricalappliances, medicine and surgery, paper andillustration, agriculture, chemistry, aircraft, fuels,and metallurgy form most interesting sections, inwhich examples are numerous enough of thetalent, skillg and energy not yet lost to our race.Synthetics and dyes are conspicuous illustrationsof recent commercial successes resting on scientificfoundations, and the exhibition of optical glassand laboratory glass ware demonstrates a brilliantdual triumph of scientist and manufacturer. The

great drug houses are well represented, showingwhat an essential equipment to the success of theirbusiness is the research laboratory. This oppor-tunity of witnessing the remarkable developments iarising out of the events of the war which havetaken place in so many departments of our greatindustries should not be lost. The bearing ofscience upon industrial success and nationalprosperity is well brought out. The exhibitioncontinues until August 5th.

THE MEDICAL AND DENTAL REGISTERS.

THE official Register of the General MedicalCouncil, which has just been issued, contains thenames of 43,926 persons, of whom 54 per cent. areon the local Register for England, 32 per cent. onthat for Scotland, and 14 for Ireland. 1077 nameswere registered in 1918, being the smallest numberrecorded for a quarter of a century except at thelow tide of 1910-11. On the other hand, the netincrease of 107 registered names at the end of1918 as compared with 1917 is quite withoutprecedent in its smallness. The previously leanestyears had some compensation, lacking in 1918, innames restored to the Register and short deathrolls. The additional names registered during1918 included 148 colonial and 16 foreign

1 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg., xxxii., 98., 1904.

diplomas, for the most part Belgian. 995 nameshave been removed from the Register during theyear, 946 on evidence of death. Only one medicalman has formally removed his name from the listas having ceased to practise.The official Dentists Register for 1919 contains the

names of 5567 dentists, of whom 13 are on thecolonial and 19 on the foreign list. Of the 5535United Kingdom dentists, 4290 hold degrees or

licences in dental surgery, and 1245 are registeredon their own declaration of being in bona-fidepractice of dentistry. Of the former, 461 haveadditional surgical or medical qualifications, of thelatter 7. The number added by registration duringthe year 1918 was 131, being one in excess ofthe figure for 1917 and barely one-half of the 1915record. 102 names were removed on evidence ofdeath, none on ceasing to practise.

EPIDEMIC ENCEPHALITIS (NONA).

A USEFUL contribution 1 to the subject ofepidemic encephalitis, from the pen of Dr. PeterBassoe, associate professor of medicine, RushMedical College, Chicago, shows that the diseasehas made its appearance in America, with similarclinical and pathological features to those fre-

quently described in this country and on thecontinent. Sainton has attempted to give a defini-tion of the disease in the following terms: "Atoxic, infective, epidemic syndrome, characterisedclinically by the triad lethargy, ocular palsies, and afebrile state, and anatomically by a more or lessdiffuse encephalitis, most marked in the greymatter of the midbrain." It may be questioned,however, whether any useful purpose is served atpresent by condensed definitions ; Sainton’s is notsufficiently explicit to be distinctive. The associa.tion of epidemic encephalitis with influenzaappears to be more than a mere coincidence, sinceit has been noted on a number of occasions. Dr.Bassoe quotes from a French source thatCamerarius, who described an influenza epidemicin Tubingen in 1718, mentioned a " sleepingsickness " in connexion with it. In 1768, Lepecqde la Cloture recorded a " coma somnolentum "

after influenza, and Ozanann (1835) mentionedepidemics of " catarrhal fever " with

"

soporosite "as having occurred in Germany in 1745, in Lyonsin 1800, and in Milan in 1802. No doubt much ofthis is vague and indeterminate; influenzal mening-itis is a different affection altogether, and what(much or little) of these epidemic conditions shouldbe classed as encephalitis lethargica is a matterdepending really on the interpretative proclivitiesof the reader. Dr. Bassae describes 11 cases in hispaper, and notes that in no one had there beendefinite preceding influenza, but suggests that theencephalitis " may be caused by a separate virus.which in order to become active must have beenin contact at one time or another with that ofinfluenza." Be this as it may, the bacteriologicalaspect of the question has been less completelyinvestigated than the clinical and pathological, andhas scarcely passed the stage of conjecture. It isan interesting observation that the brain-stem andbasal ganglia are particularly liable to be affectedby poisons, endogenous or exogenous, though whythis should be so is not at present clear. Dr. Bassoehas had the opportunity of making a completepathological examination in two cases, and hasfound the changes with which previous writers

1 Journal of the American Med. Assoc., April 5th, 1919, p. 971.