Insect Vision and Insect Sleep. 103 



infer that objects to them have the same tint as these hues of 

 the choroid ? Are they coloured as we see landscapes to be 

 when we look through a window of painted glass ? Through 

 the red, are objects beheld as if they were blazing in a fiery fur- 

 nace ; or do they appear as frigid as a snow scene in blue eyes 

 as through the blue glass ? Some purpose is served by the 

 relation. Let us see, or let us make inquiry what it may be ; if 

 it ends in nothing certain, it will, at least, be attended with 

 instruction and amusement. 



In the solar spectrum, there are rays independent of those of 

 light, which impart a sensation of heat. These calorific rays 

 are most abundant a little beyond the red verge of. the spectrum, 

 and diminish gradually towards the violet. When it was ob- 

 served, in the Conservatory at Kew, that plants suffered from 

 the scorching influence of the calorific rays through the glass 

 covering, a series of experiments were pursued to ascertain the 

 possibility of cutting off the heat-imparting rays by means of 

 tinted glass. A glass tinted of a pale yellow-green prevented 

 the permeation of the heat rays to the maximum of the calorific 

 action. The pinky hue of the light was modified, and the 

 scorching influence subdued. What they sought to accomplish 

 was effected. They obtained a properly moderated heat. 



White is the simultaneous sensation of all the prismatic 

 colours. By suppressing red, we obtain a bluish-green hue; 

 by suppressing the blue and the green, we obtain an excess of 

 yellow and red. The purest air, or clearest water, gradually 

 extinguishes, by absorption, the rays passing through them. 

 The lesser stars are visible on the loftiest mountains ; but their 

 lessened light hardly affects the sight in the stratum of the 

 atmosphere at the depth of the plain below. The natural 

 stimulus of the retina is the action of the luminous rays. Modi- 

 fications are essential where the activity of perception may be 

 allied to the conditions of diseased sight. " Many are the waves 

 and coruscations — the fiery clouds and flaming spectra which 

 haunt the amaurotic when certain morbid complications exist," 

 and when the optic nerve is peculiarly influenced, a compensa- 

 tory modification of the peculiarity in regard to tint is made by 

 the adoption of coloured glasses for the sight. We may pre- 

 sume that what is in excess in the locust is modified by the red 

 pigment of the eye, and what is superabundant in the grass- 

 hopper by the yellow and the blue. 



The eyes of insects are what are called facetted eyes. They 

 are cut in hexagonal compartments, and have the appearance 

 and the power of multiplying glasses. The outer coat is com- 

 posed of a thin plate, resembling horn. It is stiff but flexible, 

 and compact but transparent. Immediately beneath each cor- 

 neule or hexagonal compartment, that is, beneath the facets of 



