THE MICROSCOPE. 



against which the observer must be on his guard. The effects of 

 these are to surround the outline of the object with coloured fringes. 

 By limiting the illumination as far as it is practicable to the object 

 itself, so as to avoid the transmission of any light through thc^ 

 opening of the slide, except what may pass through the object, 

 this effect may be diminished or avoided. 



Indeed, for many reasons, it is advantageous to prevent any 

 light from passing through the slide, or through the opening of 

 the stage, except what is employed in illuminating the object. 

 All such light is liable to fall in greater or less quantity upon the 

 object-glass, and, passing through it, has a tendency to render the 

 image obscure and confused. For this reason, all extraneous 

 light whatever should be as far as possible excluded from the 

 space around the microscope, for all objects on which such light 

 falls will reflect a part of it, some of which may fall upon the 

 object-glass. 



50. "When the light of the sky or clouds is used, an aperture 

 may be made in a window-shutter for its admission, all the other 

 windows of the room being closed, and the light proceeding 

 from the aperture being received upon the mirror or lens, by 

 which it is directed and condensed upon the object. The light 

 of a white cloud, strongly illuminated by the sun, is generally 

 considered the best form of day-light which can be used, and 

 that of a blue serene sky the worst. Observers differ as to the 

 direct light of the sun, some maintaining that in no case what- 

 ever should it be used, while others give it a preference for 

 minute objects seen under high powers, and therefore requiring 

 intense illumination. 



The light reflected from a white wall upon which the sun 

 shines is a good source of illumination. 



51. If artificial light be used with low powers, a common sperm 

 candle will serve well enough, but means should be adopted to 

 prevent the flickering of the flame. 



An argand lamp, however, is, in all cases, preferable, as giving 

 a steady invariable light. It will be improved if good olive oil 

 be used instead of the fish oil. 



The flame produced by the liquid known as camphine is 

 especially pure and white, and well fitted for microscopic 

 researches. 



Whatever be the artificial light used, it ought to be surrounded 

 with a shade, and so placed as to fall only upon the mirror or 

 lens by which it is directed to and condensed upon the object. 



52. It is advantageous to protect the eyes of the observer from 

 extraneous light : the most simple and convenient method of 

 effecting which is by a circular blackened pasteboard screen 



48 



