466 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
wards been exposed to mercurial vapours, furnishes a very sim- 
ple example of this definition. 
As I am here obliged to give a determinate definition of light, 
I may perhaps be allowed to cast a comparative glance at the 
nature of heat, by which means its value may be better under- 
stood. The human frame is affected by both forces, by light 
as well as by heat, and in general detects their presence. But 
precisely in the same manner that it was found necessary to 
abstract from the impression of heat on the senses, so it is now 
in regard to light. Although the eye is much more sensitive to 
the latter power than the general feelings are to heat, although 
it is even capable of distinguishing the different degrees of re- 
frangibility, still there is a class of rays of light which entirely 
escapes its attention, although these rays possess a greater index 
of refraction than all the others. The human eye, therefore, is 
not sufficient for proving the presence of rays of light in a par- 
ticular case; and even among the group of visible rays it points 
out certain relations of intensity which are not confirmed by the 
majority of other bodies, and which depend either on the con- 
struction of the retina or upon the nervous membrane not being 
directly exposed to the rays of light, but placed behind some 
refracting bodies, which may easily cause an unequal absorption 
of rays possessing various periods of oscillation. 
When we abstracted from the subjective impressions of heat, 
we were at least fortunate enough to find an objective action of 
this power, which it exerts equally on all bodies; the measure- 
ment of its intensity has been the direct result. We are not yet 
so far advanced with regard to light, and there is as yet no 
possibility of measuring the intensity of rays of different refran- 
gibilities. 
Before the expanding power of heat was known, the following 
definition of the force might have been allowed, although, prac- 
tically, it would not have been very useful. Heat is that which 
when acting on particular parts of a surface modifies them so 
that they condense vapours of various kinds otherwise than 
usual, This definition would have been correct, but not very 
applicable, on account of its belonging to the nature of heat to 
expand itself on all sides, both without and within the same sub- 
stance. If light, on the contrary, affects portions of any sur- 
face, nothing of this extension is visible, a proof of which we 
often observe in the extraordinary accuracy of a Daguerre’s pic- 
