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 

 be received upon another condensing lens, by which its con- 

 vergence might be 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 be 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 

 may be 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 Qepw (thSrme) heat. 



N2 179 



