THE MICROSCOPE. 



39 



effects, are admirably made by English opticians. As they are designed 

 for their own microscopes, however, a special small condenser would 

 have to be made for such a micro- 

 scope as Hartnack's. (I cannot 

 recommend Hartnack's achro- 

 matic condenser.) Mr. Dallin- 

 ger's article l( On a New Arrange- 

 ment for Illuminating and Cen- 

 tering with High Powers," the 

 reference to which is given in 

 51, is well worthy of careful 

 perusal in this connection. Mr. 

 Dallinger and Dr. Drysdale have, 

 by their remarkable researches on 

 bacteria, shown how very tho- 

 roughly they have studied the 

 points to be attended to for the 

 proper illumination of microscopic 

 objects under high powers. 



53. Polarising Apparatus. 

 This commonly consists of two 

 Nicol's prisms. One, termed the 

 polarizer, is placed below the 

 stage ; the other, termed the ana- 

 lyser ', is placed either immediately 

 above the objective, or above the 

 eye-piece. Hartnack's polari- 

 scope is perhaps the best. The 

 analyser is placed in a special 

 eye-piece, with a graduated circle 

 and indicator to show the num- 

 ber of degrees through which the 

 prism is rotated. The ray, polar- 

 ised by the lower prism, can pass TIG. 36. How's lamp for the microscope, 

 through the upper prism, when 



the principal sections of the two are parallel ; but when they are placed 

 at right angles, the polarised ray is arrested in the upper prism, and all 

 is darkness. 



If an object be placed on the stage and the Nicols be crossed, it is 

 invisible if it refract light singly, but it appears luminous if it refract 

 light doubly ; the reason being that the object so twists the polarised 

 ray that it can pass through the upper prism. Polarised light is used 

 in the examination of striped muscle and other tissues, and various 

 crystals. 



THE METHOD OF TESTING THE MICROSCOPE. 

 54. Test Objects. Before purchasing a microscope 



