118 PRACTICAL TREATISE ON THE MICROSCOPE. 



object-glass ; the cap is then removed and the object placed 

 on the stage, and the light obtained from a white cloud or 

 bright sky, using the plane mirror; the reflector is then 

 moved to or fro till the object is best illuminated. The rays 

 of lamp or candlelight must be rendered parallel, by means of 

 the large planoconvex lens or condenser, placed with its flat 

 side near to the lamp ; the light is then reflected through the 

 illuminator by means of the plane mirror, as before. The 

 readiest way of ascertaining if the rays of light be parallel 

 and thrown in a proper direction, is to hold a card or sheet 

 of paper on the mirror, and adjust the distance of the con- 

 densing lens from the lamp till the circle of light is of the 

 same diameter as the lens employed, and occupies the centre 

 of the mirror. The apparatus just described is made of 

 various sizes ; but as a very intense light is required for this 

 principle of illumination, it is advisable that the reflector 

 should be of as large a size as the stage fittings will admit ; 

 for if we double the diameter of the reflector, we obtain four 

 times the quantity of light, the areas of circles being to each 

 other as the square of their diameters. 



Prism. — M. Dujardin, to whom we are indebted for the 

 achromatic condenser, found that to produce the best efiects, 

 a prism of glass, of the form represented by fig. 63, should be 

 used with it, instead of a mirror, a re- 

 presents a short piece of brass tube, b a 

 glass prism, connected by screws to the 

 tube, a, by two supports, c c. The tube is 

 made to slide upon the end of the con- 

 denser, and to turn upon it in such a man- 

 ner, that, in whatever position the lamp or 

 white cloud may be, the prism may be 

 adjusted to it ; the revolution of the prism 

 being performed upon the screws, the extre- 

 pio-. 63. mities of which are conical and fit into cor- 



responding depressions in the side of the 

 prism. This instrument has some few advantages over the 

 plane mirror : the quantity of light is greater, and all test 

 objects in which delicate markings exist, may be shown to 



