OP THE MICROSCOPE. 169 



pieces, thus cutting off all the light which passes through the 

 microscope, except that which actually serves to illuminate the 

 object. 



It will also be found of great importance to secure perfect 

 purity in the special illumination employed. Thus, if we are 

 examining an object by transmitted light, it always detracts 

 from the clearness and beauty of the image if light is reflected 

 from its surface. It is, therefore, of advantage to shade the 

 object by means of a small tin, brass or pasteboard shade, at- 

 tached to the stage so as to prevent any light from the lamp 

 from falling on the object. 



A difficulty which frequently occurs to young microscopists 

 consists in the almost impossibility of securing a field of view 

 equally illuminated in all parts. Assuming that the mirror is 

 in proper position, and that there is nothing to shade any part, 

 it will in general be found that the difficulty arises from the 

 fact that the mirror throws images of the lamp, etc., upon the 

 object. Sometimes this is very distinctly seen; the shape of 

 the flame can be clearly distinguished, and the metal portions 

 of the lamp appear as dark shades. The cause is that the lamp 

 is at the exact distance at which the mirror forms an image of 

 it on the upper surface of the slide, just as a lens, held in 

 front of a white wall, will throw an inverted image of a lamp 

 or candle on the wall, provided the relative distances of the 

 wall, lens and candle are properly adjusted. The remedy is 

 very simple; move the lamp either towards the microscope or 

 away from it, as may be most convenient. 



As previously stated, the character of the illumination af- 

 forded by a mirror, and by a white surface placed at a short 

 distance from the object, are appreciably different. A very 

 pleasant method of illuminating transparent objects consists in 

 the use of a plate of plaster of paris. Its whiteness is probably 

 as pure as that of any other substance, and it is easily procured. 

 The plate we use was cast in the cover of an old tin box, half an 

 inch deep and three inches in diameter. Aflat surface was 

 secured by casting it upon a board. If cast on glass or metal, 

 the surface is glazed and shiny, which is bad. Instead of plas- 

 ter, fine white paper or cardboard may be used. Such surfaces 

 must not be glazed, and they should be kept scrupulously clean. 



