Contributions to the Physiology of Vision. 535 



changing the position of the eye, so that its image may fall on 

 another part of the retina ; stars also, which, from the feeble- 

 ness of the light they emit, are ordinarily invisible, may be 

 made apparent by the same means ; and if the statement of 

 Majendie and Desmoulins be correct, stars may be thus made 

 to appear in full daylight *. 



Dr. Brewster has described several analogous cases of the 

 disappearance of visible objects t ; and the following experi- 

 ments of the late Benedict Prevost of Geneva (published post- 

 humously in the ( Mem. de la Soc. de Phys. et d'Hist. Nat.,' 

 t. iii. 2d part) will afford an additional instance, and will at 

 the same time prove to us that the employment of very simple 

 means may greatly assist our powers of observation with regard 

 to a variety of optical phenomena, in which the feebleness of 

 the objects might otherwise be deemed an obstacle to success- 

 ful inquiry. 



On an appearance of Decomposition of White Light, by the Motion of 

 the Body which reflects it. In a chamber sufficiently dark, into which 

 a ray of the sun penetrates, move a rectangular piece of white card, about 

 two inches in breadth, backwards and forwards, as if you would cut this 

 ray nearly perpendicularly to its axis. 



At the moment the white card traverses this axis, the eye which regards 

 it evidently receives from this object a white light, as if the card remained 

 stationary at this place. But it happens, however, that the disc, illumi- 

 nated by the ray, the section of which it represents, appears coloured; 

 it is white only in the centre. The very small white space which sur- 

 rounds the centre, changes to violet, deepening as it recedes. The violet 

 spot is surrounded by a zone of a deep indigo colour, very distinct and 

 well denned, and exactly resembling the colour of the heart's-ease (viola 

 tricolor). Around this indigo zone is a zone of greenish-yellow, equally 

 well denned ; then, surrounding it externally, a red tint. If the observer 

 be very attentive, and seize the most favourable moments and situations, 

 it will be seen that the white ray reflected by the disc has been decom- 

 posed, as it would have been by the prism, into seven principal colours, 

 arranged nearly in the same order. 



* During day-time, in an unclouded sky, the light of the stars, which is but one 

 sixty-fourth of the luminous splendour of the atmosphere, is insensible to our eyes. 

 In general, any body projected and immoveable on a plane with which it has this 

 same degree of luminous intensity, is invisible. But if by a displacement, either 

 of the body upon this plane, or of the image of a star in a telescope, it is made to 

 pass over a certain arch, repeated on the retina by the displacement of tin 

 of its rays, this body or the image of the star becomes visible. (Majeudie ct Des- 

 moulins, Anatomic des Systemes Nerveux des Animaux a vertebres. Tom. 

 ii. p. 670.) 



f Edinburgh Journal of Science, No. IV. 



2 N -' 



