Royal Institution. 309 



This head commences with the quotation of a passage from Sir 

 David Brewster's 'Optics' which he offered towards an explanation 

 of the difficulty of seeing a very faint star by direct vision ; and it is 

 shown that the retina is not liable, as Brewster imagines, to be 

 thrown into a state of " undulatory" perception by our looking 

 through the teeth of a "fine comb" or through a single " narrow 

 aperture." The paper points out that the effect observed in these 

 circumstances is produced by our looking near the edge of any body 

 whatever, provided, and only then, that the object move, be it never 

 so little, across the eye's axis. It shows that the same effect is pro- 

 duced by light radiating from a point, by a flame, by lenses, curved 

 reflectors, whilst they are in the act of moving across the eye's axis ; 

 or by the movement of the eye itself, merely in relation to the light 

 entering it, — even the naked eye along the sky. The effect pro- 

 duced is shown to be simply owing to this ; that the retina, under 

 such action, ceases to perceive in the spaces corresponding to its 

 blood-vessels and capillaries, so that they completely display them- 

 selves in the semblance of black bodies (or lines) ; and the law is 

 arrived at, that the images of external points which are painted on 

 the vessels and capillaries are not perceived when the retina loses 

 light from one point of space and receives light from another point of 

 space within a certain interval of time, or that the percipient points 

 lying in front of the vessels require a certain time to perceive. A 

 physiological hypothesis is suggested to account for this phseno- 

 menon, on the presumption that the " radial fibres," which project 

 from the layer of rods and cones and end in the limitary membrane, 

 are the ultimate percipients of light. 



It is pointed out how wonderfully close we may find the corre- 

 spondence between the microscopical and optical anatomy of the 

 retina. Each pair of identical fibres of the two optic nerves must 

 be regarded as one nerve. Another supposed anomaly to the sim- 

 plicity of nervous action being explained on anatomical principles, a 

 statement of ordinary optical nervous action is made, and a summary 

 evinces how the anomalies in visual experience are due to the com- 

 plex additions to a simple organ of sight. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



Feb. 2, 1855. — "On the Pendulum-experiments lately made in the 

 Harton Colliery, for ascertaining the mean Density of the Earth." 

 By G. B. Airy, Esq., F.R.S., Astronomer Royal. 



The Lecturer commenced with remarking that the bearing of the 

 experiments, of which he was about to give a notice, was not 

 limited to their ostensible object, but that it applied to all the bodies 

 of the solar system. The professed object of the experiments was 

 to obtain a measure of the density of the earth, and therefore of the 

 mass of the earth (its dimensions being known) ; but the ordinary 

 data of astronomy, taken in conjunction with the laws of gravitation, 

 give the relative proportions of the mass of the earth to the masses of 

 the sun and the principal planets ; and thus the determination of the 



