262 THE USE OF THE MICROSCOPE 



used. This may be tried with \ariou8 (halonis. Fine 

 test objects may also be tried with a tourmahn (or Nicol 

 prism) before the small source of light; or, instead, with a 

 tourmalin over the eyepiece. The tourmalin, or Nicol 

 prism, should be rotated until it is in the right azimuth 

 with regard to the particular set of lines in question.' This 

 is because light strongly diffracted by a line is polarized 

 in one direction, and the tourmalin cuts out more or less 

 of the other Hght. (Stump, 129.) 



For the use of a narrow beam of oblique light see Spitta 

 (127). This method is not recommended, since the images 

 are only partial. 



In the writer's experience, oil-immersion objectives of 

 1.3 aperture which allow of a nine-tenths cone on the dots 

 of a fine specimen of Surirella gemma in hyrax, and at the 

 same time permit a fairly sharp enlargement of 1,800 

 times, also give brilliant images of all objects at the maxi- 

 mum useful magnification of 1,250. (The writer found only 

 two such objectives of optimum quality out of six high- 

 power apochromatic and fluorite objectives he used in his 

 work.) 



32. Diaphragm in Drawtube. — With a large source 

 of light, an oil-immersion objective and a ^fo cone, on a 

 well-stained object with the monocular microscope, try 

 the effect of a 14-millimeter diaphragm in the drawtube, 

 about 1 centimeter below the longest eyepiece. Try the 

 same with the source of light equal to the source-field. 

 (Nelson.) 



33. Cover-glasses of Excessive Thickness. — Remove 

 the concave achromatic amplifier from the lower end of the 

 Siedentopf (or other) binocular attachment, and fit it into 

 the lower end of the drawtube of the monocular microscope. 

 Use the 40 objective of 0.65 (or 0.85) aperture. Fit a 

 cover-glass 0.10 millimeter (or more) thick on the wedge- 

 shaped cover-glass of the Abbe test plate by a little immer- 

 sion oil between. Find the best cover thickness for the 

 combination, by the definition, and by the star test. 

 (The position of the drawtube may be altered.) The 



