Insect Sight and the Defining Power of Composite Eyes. 85 



immersion, was heated in hydrogen. The action would be repre- 

 sented by the formula 



4Cu + S0 3 = CuS + 3CuO. 



The above experiments prove that nearly the whole effect which 

 is observed when copper is immersed in a solution of sulphate of 

 copper or sulphuric acid is due to the presence of oxygen in the 

 solution. 



February 1, 1894. 



Sir JOHN EVAN'S, K.C.B., D.C.L., LL.D., Vice-President and 

 Treasurer, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



I. " Insect Sight and the Defining Power of Composite Eyes." 

 By A. MALLOCK. Communicated by LORD RAYLEIGH, 

 Sec. U.S. Received November 28,. 1893. 



The optical arrangement of the simple eyes of Vertebrates is well 

 understood, but as regards the action of the composite eyes of Insects 

 and Crustacea less certainty has hitherto prevailed. 



In the former class of eye a single lens, or its equivalent, forms an 

 image on a concave retina, built up, as a sort of tesselated pavement, 

 of the sensitive terminations of the fibres of the optic nerve, and, if 

 the lens is perfect and the pupil large enough, the definition is limited 

 by the distance apart of the nerve-terminations, for, in order that two 

 objects may appear as two to the eye, they must subtend at least such 

 an angle that their images as formed by the lens shall not fall on the 

 .same nerve-termination. 



In the human eye the distance between the sensitive points on the 

 retina is such that it subtends about a minute of arc at the optic 

 centre of the lens, and in good eyes the optical part of the apparatus 

 is sufficiently perfect, to allow of this degree of definition being 

 attained over a small part of the field of view. 



For reasons, however, which will be given presently, such defini- 

 tion as this is not to be looked for in composite eyes. 



The general plan on which all composite eyes are constructed is 

 that of a convex retina having a separate small lens in front of each 

 sensitive part, together with an arrangement of screens which allows 



VOL. LV. H 



