818 SUMMARY OF CURRENT RESEARCHES RELATING TO 



crystalline) of the eye and not in the cerebro-retinal centres, for on ex- 

 amining a horizontal line through a small hole from • 8 mm. to 1 mm. 

 one of the images only was seen, but both became successively visible on 

 displacing vertically the hole, and on impressing a suitable velocity on 

 this displacement an undulatory appearance was given to the line. No 

 phenomena of double refraction were observed on examining with a 

 nicol. In certain instances triplopia was also obtained, the third image, 

 however, being very pale. The energy and duration of the disturbance 

 was naturally found to vary with the length of microscopic observation, 

 and its disappearance was progressive and continuous. Thus, on one 

 occasion, when the author began work (observation of diatoms) at 10 

 in the morning, at 10.30 there was diplopia, and at 11 triplopia. The 

 separation of the images was then measured and amounted to 4' for the 

 second and 8' for the third. At noon the triplopia had disappeared, 

 but diplopia still remained. 



Apart from microscopic observation diplopia was also found to 

 result from observing across a small hole a phenomenon difficult to catch 

 at the moment of its appearance or disappearance in a very limited field, 

 and also in some degree from examining ophthalmometric images. 



M. J. J. Landerer,* in reference to M. Leroy's note, claims to have 

 been the first to call attention to this phenomenon, and adds the fol- 

 lowing remarks concerning it : — 



(1) Although the effort experienced by the eye seems to be of the 

 same nature for microscopic as for telescopic vision, yet the disturbance 

 consequent in the closed eye is much more marked in the first case 

 than in the second. This difference is maintained not only when the 

 telescopic object is so difficult a one to catch with a telescope of 108 mm. 

 aperture as the shadow of the second satellite of Jupiter as it is pro- 

 jected on the edge of the planet, but also when the image has considerable 

 brightness, as when the granulation of the sun's surface or the spots are 

 examined through only a slightly blackened glass. This difference is 

 not due to the different inclination of the head in each case, for it still 

 persists when the telescopic observation is made by means of the bent 

 eye-piece. 



(2) That during microscopic observation there is a crossing of the 

 optic axes of the two eyes, producing an effect similar to that of strabism, 

 is proved by the fact that by giving them this disposition, and then 

 applying the eye to the eye-piece, the image is seen with perfect dis- 

 tinctness. 



It is the simultaneous effort of both eyes which explains the dis- 

 turbance undergone by the closed eye. But as this effort acts in an 

 unconscious way, and has struck no one's attention, it has been supposed 

 that there is here only an effect of accommodation producing the defi- 

 nition of the image at the distance of the punckim proximum. The 

 above facts appear really to show that this is not the case, or, at least, 

 that there is no reason to affirm that the image is not defined at the 

 distance of distinct vision properly so called. 



Amplifying Power of the Microscope.t — Dr. L. Didelot has applied 

 to the Microscope the notions and formulae concerning the amplifying 



* Comptes Eendus, cix. (1889) pp. 74-5. 



t Didelot, L., ' Du Pouvoir amplifiant du Microscope : determination theorique 

 et expf'riraentiile suivie d'une table a quatie de'cimales des inverses des 1000 premiers 

 nombres Ue 0-01 a 10 '00,' 2nd ed., 8vo, Paris, 1887, 86 pp. (1 pi.). 



