92 OF THE SENSES OF 
their size, shape, or elasticity, are susceptible of 
vibrating in times exactly equal to those at which 
the impulses are repeated. Thus, it is easy to 
conceive how the limits of visible colour may be 
established ; for, if there be no nervous fibres in 
unison with vibrations, more or less frequent than 
certain limits, such vibrations, though they reach the 
retina, will produce no sensation. Thus, too, a single 
impulse, or an irregularly repeated one, produces no 
light ; and thus, also, may the vibrations excited in 
the retina continue a sensible time after the exciting 
cause has ceased, prolonging the sensation of light, 
(especially of a vivid one,) for an instant in the eye. 
We may thus conceive the possibility of other 
animals, such a8 insects, incapable of being affected 
with any of our colours, and, receiving their whole 
stock of luminous impressions from a class of vibra- 
tions altogether beyond our limits, as Dr Wollaston 
has ingeniously imagined, (we may almost say, 
proved,) to be the case with the perceptions of 
sound.” * 
PAIRING OF LEPIDOPTEROUS INSECTS, 
In almost all insects, there is great variety in 
the colour of the males and females ; and in many 
they are so different in form, as to be taken for 
different species. 
In butterflies, the males are usually of a brighter 
* Encyclopedia Metropolitana, Article Licut. 
