ILLUMINATING APPARATUS. 
which is cut at right angles to its axis by the slider upon which 
the object is fixed. 
Let c c, fig. 1 , be the condensing lens; let f be the focus to 
which the rays would be made to converge, but being intercepted 
by the slider s s, they are collected upon the small circular opening 
o o in the slider, and in this circular opening the small microscopic 
object to be exhibited is mounted between two thin plates of glass. 
Now, it is evident, that the intensity of the light thus projected 
upon the object will be greater than that with which it would be 
illuminated without the interposition of the lens c c, in the exact 
proportion of the surface of the lens c c to the surface of the cir¬ 
cular opening o o. Thus, for example, if the diameter of the lens 
c c be 5 inches, and the diameter of the opening o o half an inch, 
the diameter of the lens will be 10 times, and, therefore, its sur¬ 
face 100 times greater than that of the opening o o. In that case 
the object would be illuminated with a light just 100 times more 
brilliant than if the sun’s light fell directly upon it, without 
passing through the lens c c. 
It is found convenient in some cases to condense the light by 
means of two lenses. The cone of rays proceeding from c c might 
he received upon another condensing lens, by which its con¬ 
vergence might he increased. The advantage of this arrangement 
is that the distance of the object from c c, and therefore the length 
of the microscope, is rendered less than it otherwise would be. 
5. There is, however, one practical inconvenience to he guarded 
against in this arrangement. The lens c c, which condenses the 
sun’s light upon the object, also condenses its heat, and if the 
same object be exposed in the instrument for any considerable 
time, it would thus be injured or destroyed. This inconvenience 
maybe obviated by the interposition of certain media, which, 
while they are pervious to the sun’s light, are impervious to its 
heat; such media are said to be athermanous.* 
* From the Greek negative a (a) and Ocpfirj (therme) heat. 
n 2 179 
