03? THE 5IICKOSCOPE. 115 



Ifc forms a prominent feature in the New American Model, as 

 well as in all first-class microscopes. 



In its simplest form it consists simply of a short tube or ring, 

 which is attached to the under side of the stage, and is fitted to 

 receive polarizers, paraboloids, condensers, etc. It should be 

 made removable, so that it may offer no obstacle to the employ- 

 ment of the most oblique illumination. . 



In the New American Model it is attached to a swinging arm, 

 so that the illuminating apparatus, which it carries, may be 

 placed at any angle with the optic axis of the microscope. 



When a mere ring is used for receiving the accessory pieces 

 of apparatus, the latter are adjusted by simply pushing them 

 up or down, but a much better plan is to have the ring itself 

 moveable, as is the case in the instruments shown in Plates III, 

 IV, V, and VI. The accessory is then placed in the ring, and 

 the latter may be slid up or down the carrying bar, so as to 

 allow of proper adjustment. A still better plan is that shown 

 in Plate I, where the distance of the sub-stage from the stage 

 may be adjusted by means of a rack and pinion. 



In any case the sub-stage itself, or the apparatus which it 

 carries, should have facilities for accurately centering the 

 various pieces of illuminating apparatus. 



Tlie Mirror. The mirrors employed for illuminating 

 microscopic objects are either plane or concave, and in the 

 better class of instruments both kinds are provided, while the 

 cheaper forms of the microscope have only the concave mirror. 

 The plane mirror reflects the light just as it falls on it that is 

 to say, divergent rays (as from a lamp) remain divergent after 

 reflection, and parallel rays (as sunlight or rays from the bull's- 

 eye condenser) remain parallel. The concave mirror, on the 

 other hand, causes parallel rays to converge and meet at a point, 

 while other rays are rendered either less divergent, parallel or 

 convergent, as the case may be. It is very important that the sur- 

 face of the mirror should be accurately formed, and therefore in 

 all good microscopes they are made of glass, which has been 

 accurately ground and polished. Blown or cast glass will not 

 answer. And as the quality and quantity of the light is greatly 

 affected by the reflecting surface, the best mirrors are silvered 



