792 THE EYE AND VISION [CH. LVI. 



3. The curve of retinal excitation, like the muscle curve, rises not abruptly but 

 gradually to its full height, and on the cessation of the stimulus takes a measurable 

 time to fall again, the retinal impression outlasting the stimulus by about one-eighth 

 of a second. 



4. With comparatively slow intermittent excitation, the phenomenon known as 

 flicker takes place ; this may be shown by the slow rotation on Maxwell's machine 

 of a disc painted with alternate black and white sectors. This roughly corresponds 

 with what in a muscle is called incomplete tetanus. 



5. When the rate of stimulation is increased, as by increasing the speed of rota- 

 tion of the disc just alluded to (say to twenty or thirty times a second) the resulting 

 sensation is a smooth one of greyness. This fusion of individual stimuli into a con- 

 tinuous sensation, does not by any means correspond to the complete tetanus of 

 muscle, for the resultant sensation has a brightness corresponding not to a summa- 

 tion of the individual fusing sensations, but to a brightness which would ensue if the 

 stimuli were spread evenly over the surface of the disc (Talbot's Law). 



The Ophthalmoscope. 



Every one is perfectly familiar with the fact, that it is quite im- 

 possible to see ihefundu or back of another person's eye by simply 

 looking into it. The interior of the eye forms a perfectly black 

 background.* The same remark applies to the difficulty we experi- 

 ence in seeing into a room from the street through the window unless 

 the room is lighted within. In the case of the eye this fact is partly 

 due to the feebleness of the light reflected from the retina, most of it 

 being absorbed by the retinal pigment ; but far more to the fact that 

 every such ray is reflected straight to the source of light (e.g., 

 candle), and cannot, therefore, be seen by the unaided eye without 

 intercepting the incident light from the candle, as well as the 

 reflected rays from the retina. This difficulty is surmounted by the 

 use of the ophthalmoscope. 



The ophthalmoscope was invented by Helmholtz ; as a mirror for 

 reflecting the light into the eye, he employed a bundle of thin glass 

 plates; this mirror was transparent, and so he was able to look 

 through it in the same direction as that of the rays of the light it 

 reflected. It is almost impossible to over-estimate the boon this 

 instrument has been to mankind ; previous to this in the examina- 

 tion of cases of eye disease, the principal evidence on which the 

 surgeon had to rely was that derived from the patient's sensations ; 

 now he can look for himself. 



The instrument, however, has been greatly modified since Helm- 

 holtz's time ; the principal modification is the substitution of a con- 

 cave mirror of silvered glass for the bundle of glass plates ; this is 



* In some animals (e.g., the cat), the pigment is absent from a portion of the 

 retinal epithelium ; this forms the Tapetum lucidum. The use of this is supposed to 

 be to increase the sensitiveness of the retina, the light being reflected back through 

 the layer of rods and cones. It is certainly the case that these animals are able to see 

 clearly with less light than we can, hence the popular idea that a cat can see in the 

 dark. In fishes a tapetum lucidum is often present ; here the brightness is increased 

 by crystals of guanine. 



