396 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



doubtedly nearly related to the compound eye, a subcorneal lens exists. In these it 

 consists of three segments. A similar lens is seen in the semicompound eyes of isopods, 

 and is regarded by Dr. Grenacher as a cone in a highly modified condition. It 

 consists of only two segments instead of three or four. 



The cornea and oil-lens together have a very short focus. The picture formed by 

 these structures is usually from five to ten micromillimetres behind the posterior surface 

 of the lens. 



Very great differences of opinion have obtained credence concerning this image since 

 the publication of Gottsche's paper. Many authors, and quite recently Exner *, have 

 denied that any images are formed by the eye when the crystalline cones are in situ. I 

 have a specimen of the cornea and cones of a moth (Smerinthus populh), mounted in 

 balsam, in which distinct images are formed in the interior of the crystalline cones. In 

 Exner's experiment the crystalline cones were surrounded by a lowly refracting fluid. 

 As the images, according to my calculations, are formed near the focal plane of the 

 inner ends of the crystalline cones, rays emerge from the inner extremities of the cones 

 as parallel rays, or at least approximately parallel. The image cannot therefore be 

 observed by the high powers of a microscope. Exner is undoubtedly quite right when 

 he says that no image can be observed when the cones are in situ and surrounded by a 

 lowly refractive fluid ; but it by no means follows that none is formed. Indeed the 

 structure of the eye is such that an image must be formed, and it cannot be neglected in 

 working out the manner in which the eye acts as an organ of vision. 



The focal length of the corneal lenses of the Fly (Jfusca vomitoria) has been given by 

 me from actual measurement as ^q of an inch f . I have since found that the results 

 obtained by measurement are always greater than those arrived at by calculation ; this is 

 partly due to the rapid loss of refractive power immediately after death, but also to the 

 fact that any moisture adhering to the inner aspect of the lenses forms a concave surface 

 as it is attracted by the inflected margins of the corneal facets. I believe that the only 

 satisfactory results are those arrived at by calculation. The greatest difficulty is the 

 estimation of the refractive index. This is often considerably greater than that of 

 Canada balsam, which is sufficiently proved by the formation of a subcorneal image 

 when the cornea and cones of a moth are entirely immersed in it. 



Small fragments of glass in water have nearly the same brightness as the cones and 

 lenses of a moth in balsam. By reducing the illumination of the field of the microscope 

 both were just visible with the same illumination, which indicates a refractive index, for 

 the cones and lenses, of 1*8 nearly. Assuming the refractive index to be between 1-5 



and 2-0, the focus by the formula ( ^3)7x7=/ will lie between r and ' 2 , when r—r'. 



the value of the fraction ^-p varies between 10* and 20* in different insects which I have 



examined. Therefore the focal length of the corneal lens is from five to twenty micro- 

 millimetres, and falls well within the crystalline cone, and the rays frequently emerge 



* Biol. Centralblati, i. p. 280. 

 t Phil. TraDs. I.e. p. 585. 



