780 REPJET— 1887 



of the centre of tbe retina being tlius disproved. Tliey are approximately of such 

 size as if seen by a one-eighth objective, and are red in colour when best seen. They 

 appear under various circumstances of optical curvature, varying from far accom- 

 modation to the microscopic accommodation (effected, unlike tlie normal accommo- 

 dation, by alteration of the curvature of the cornea itself) which is possible to the 

 myopic eye when half-closed and s'^ueezed by the eyelids ; and the condition of 

 their perception seems to be the absence of other clearly dehned images from the 

 retina, or perhaps from the combined field of vision — a condition fultilled under 

 the following circumstances: — (1) Far accommodation when the eye is confronted 

 by a near object ; (2) near accommodation when the eye is confronted by a far 

 object; (.'<) oblique vision of a briglit source of light; (4) slipping the eyes into 

 a position in which the visual axes are parallel, so that the images of the two eyes 

 blur each other. The circulation of corpuscles observed under these conditions 

 is in all respects normal, its rapidity undergoing a variation with the rapidity of 

 the pulse, as described by Vierordt. 



Although the determining condition of the perception of images of the blood- 

 corpuscles seems to be absence of other definite images from the retina, yet these 

 images are, of course, brought into view and removed from it by alterations of 

 focus in the eye. From the wide limits of accommodation under which the images 

 are visible, as above stated, it will be understood, however, that they do not 

 depend, except in a partial and indirect manner, upon accommodation brought 

 •about in the normal way. The delicate focussing by which they are attained is 

 ■due, at least in the majority of instances and in the case of the best images, not to 

 ordinary accommodation, but to what ma}' be called retinal accommodntion, viz., a 

 backward movement of part of the retina accomplished directly by the muscles 

 ■acting on the eyeball. The existence of such a mode of accommodation, which 

 has been discussed by Professor Silvanus Thompson in another connection, is 

 shown by the manner in which the images of blood-corpuscles occur. The major 

 part of the circle of distinct vision is occupied by images of corpuscles, while 

 its margin presents the image of some e.xterior object, of which the middle is 

 •wanting — a condition which corresponds exactl}^ with the above-named theory 

 that a portion of the retina is pulled back out of its place, a local alteration of 

 its curvature being thus effected. The difficulty of explaining the manner in which 

 images of the blood-corpuscles occur has been greatly increased by the fact that 

 the strain of the muscles in producing this ' retinal accommodation ' gives rise to 

 pressure images of blood-corpuscles, which are confused with normal images if 

 their different lustre is not carefully noted. 



3. Rim of the /ow« centralis. 



4. llim of the blind spot. 



*5 and 6. Two layers, one of small, brilliantly transparent, and rovmd. the other 

 of larger, more irregular, and apparently reddish bodies, whicb probably, from their 

 relative sizes and distribution wath reference to the fovea centralis, correspond 

 respectively to the peripheral and the proximal nuclear layer. 



II. Entoptical images resulting from pressure, automatic or artificially applied : 

 •distinguished by tlieir metallic lustre ; by their colour, varying from pale yellow 

 through pale green to electric blue or purple, according to the intensity of stimu- 

 lation, possibly complicated with other causes ; and by tlieir visibility in the dark, 

 although they are usually rendered more intense by the presence of the normal 

 stimulus of light. 



1. A series of images beginning with the large nerve-branches radiating from 

 the blind spot, and extending finally to the terminal branchlets in the fovea centralis. 



2. A second series, passing from the peripheral part of tbe retina to the 

 proximal, beginning wich an image of the outer nuclear layer,* and proceeding to 

 an image of the ganglionic layer.* 



3. A central grating, composed of sets of bright bars at right angles with one 

 another, indicating structure of some kind in the fovea centralis. 



4. Pressure images of blood-corpuscles, in the form of discs usually, often of 

 ■dots, and occasionally of rings. Confusion of these, visible in darkness and 

 favoured by unhealthy conditions, with the normal images of blood-corpuscles, 



