TRANSACTIONS OF THE SECTIONS. 29 



On the Compensation of Impressions moving over the Retina. 

 By Sir David Brewstee, K.H., F.B.S. 



The author stated that when, in railway travelling, they looked at the lines which 

 the stones or gravel or other objects formed in consequence of the durations of their 

 impressions on the retina, and quickly transferred the eye to the same lines further 

 back, where the velocity was slower, the stones or gi-avel or other objects would, 

 for an instant, be distinctly seen, just as rapidly revolving objects are seen in the 

 dark when they are illuminated by an electric ilash or the light of an exploded 

 copper cap. A similar, but not the same, phenomenon will be seen when we look 

 at the moving lines through a slit and quicMy look away from the slit, so that the 

 lines may be seen by indirect vision on a pai't of the retina not previously im- 

 pressed. This class of phenomena may be best studied with a rapidly revolving 

 disc, by quickly ti-ansferriag the eye from the lines on the marginal part of the disc 

 to those near the centre of rotation, where the velocity is less. When the mai-- 

 ginal velocity is gi'eatest, the point of compensation is nearest the centi-e, as might 

 have been expected from the experiment in a railway carriage ; but what could 

 not, he thought, have been anticipated, was that the point of compensation was not 

 in the same radius as the point to which the eye was first directed. The author 

 explained this statement by means of a diagram which was exhibited. He had not 

 been able to see the point of compensation close to the centi-e of rotation, where it 

 doubtless must be, with a certain velocity, so that its locus must be in a curve. 



On the Optical Study of the Retina. By Sir David Bkewsteh, K.H., F.R.S. 



There were two structures in the retina (hexangular and quadrangular) that could 

 be exhibited by optical means, the one by the successive impulses of light, and the 

 other by the action of faint light entering the eye, or produced within it, either 

 from the duration of a luminous impression, or from a local pressm-e upon the retina. 

 The first of these structures was best seen by the light of a white cloud, through 

 the slits or apertures of a revolving disc, placed midway between its circumference 

 and its centre of rotation, in order to protect the eye fi'om light which did not pass 

 through the slits. When the disc revolved rapidly the field of view exhibited 

 neither colour nor structure, but merely a diminution of light. When the velocity 

 had reached a certain point, the field of vision became yellowish white, then yellow 

 and bluish. Occasionally the yellow had the form of a rectangular cross, between 

 the branches of which were four dark spaces. With a diminished velocity the whole 

 field became unifonuly blue, and was now covered with the hexagonal pattern 

 formed by deep-black fines, the lines being darker at the place of the foramen cen- 

 trale. As there are no fewer than eight different layers in the retina, it was of great 

 importance to ascertain the functions which they individually perfoi-med in con- 

 veying visual impressions to the brain, and it was only by optical means that this 

 inquiry could be conducted. The anatomist had ably perfoimed his part with the 

 aid of the microscope, and it was probably from the improvement of this instrument 

 chiefly that we could expect any further discoveries, vmless the morbid anatomy of 

 the retina should connect certain imperfections of vision with the condition of cer- 

 tain layers of the membrane. When the eye was left in darkness, by the sudden 

 extinction of a light, there were several points at the margin of the retina which 

 retained the light longer than the rest. Ihere could be no doubt that these effects 

 were produced by structural differences. In the case of the foramen the difference 

 had been recognized by the anatomist, and was proved by the remarkable pheno- 

 menon of Haidinger's brushes, and by other optical facts, such as the instability and 

 superior brightness of oblique impressions on the retina. We had, consequently, 

 an optical principle which enabled us to explain the quadrangular structure he had 

 referred to. It was not improbable, when we looked at the complete structm-e of 

 the retina, and even of its individual layers, that the structure of each of them 

 might be exhibited optically. 



On Binocular Lustre. By Sir Davtd Bbewsteb, K.H., F.R.S. 



The author commenced hj stating that some years ago it was observed by Pro- 

 fessor Dove that when the nght and left eye figures of a pyramid, or other mathe- 



