CH. Ill] CHOICE OF A MICROSCOPE 99 



(3) that the stage is shaded; 



(4) that the tube-length of the microscope is that for which the 

 objectives were corrected (Ch. IX). 



(5) that the preparation is clean and gives a good image with the 

 microscope with which one is familiar. If all the precautions have 

 been taken and still a good image cannot be obtained one should get 

 some more expert friend or the makers to show wherein the trouble 

 lies. 



LABORATORY AND HIGH SCHOOL COMPOUND MICROSCOPES 



168. Optical parts. A great deal of beginning work with the 

 microscope in biological laboratories is done with simple and inex- 

 pensive apparatus. Indeed if one contemplates the large classes in 

 the high schools, the universities, and medical schools, it can readily 

 be understood that microscopes costing from $25 to $50 each, and mag- 

 nifying from 25 to 500 diameters, are all that can be expected. But 

 for the purpose of modern histological investigation and of advanced 

 microscopical work in general, a microscope should have something 

 like the following character: Its optical outfit should comprise 

 dry objectives of 50 mm., 16-18 mm. and 4 mm. equivalent focus. 

 There should be present also a 2 mm. or 1.5 mm. homogeneous immer- 

 sion objective. Of oculars there should be several of different power. 



Even in case all the optical parts cannot be obtained in the begin- 

 ning, it is wise to secure a stand upon which all may be used when 

 they are finally secured. 



169. Objectives. Achromatic objectives will serve all ordinary 

 purposes. For photo-micrography and the finest work where the 

 color values are of essential importance, the apochromatic objectives 

 and compensation oculars should be obtained, if possible, although 

 even in photography and the most difficult fields of microscopy the 

 modern achromatic objectives give excellent results. 



170. Mechanical parts or stand. The stand should be low 

 enough so that it can be used in a vertical position on an ordinary 

 table without inconvenience (fig. 25, 38); it should have a jointed 

 (flexible) pillar for inclination at any angle to the horizontal. The 

 adjustments for focusing should be two, a coarse adjustment or 



