CH. II] LIGHTING AND FOCUSING 



75 



Indeed if one contemplates the large classes in the high schools, the universi- 

 ties and medical schools, it can be readily understood that microscopes costing 

 from $25 to 50 each and magnifying from 25 to 500 diameters, are all that can be 

 expected. But for the purpose of modern histological investigation and of ad- 

 vanced microscopical work in general, a microscope should have something 

 like the following character: Its optical outfit should comprise, (a) dry objec- 

 tives of 50 mm. (2 in.), 16-18 mm. (% in.) and 3 mm. (>£in ) equivalent focus. 

 There should be present also a 2 mm. ( r \ in. ) or r.5 mm. (j l s in. ) homogeneous 

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

 A centering substage condenser, and an Abbe camera lucida are also neces- 

 sities, and a micro-spectroscope and a micro-polarizer are very desirable. 



Even in case all the optical parts cannot be obtained in the beginning, it 

 is wise to secure a stand upon which all may be used when they are finally 

 secured. 



As to the objectives. The best that can be afforded should be obtained. 

 Certainly at the present, the apochromatics stand at the head, although the best 

 achromatic objectives approach them very closely. 



I 133. 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 inconven- 

 ience; it should have a jointed (flexible) pillar for inclination at any angle to 

 the horizontal. The adjustments for focusing should be two, — a coarse ad- 

 justment or rapid movement with rack and pinion, and a fine adjustment by 

 means of a micrometer screw. Both adjustments should move the entire tube 

 of the microscope. The body or tube should be short enough for objectives 

 corrected for the short or 160 millimeter tube-length. It is an advantage to 

 have the draw-tube graduated in centimeters and millimeters. The lower end 

 of the draw tube and of the tube should each possess a standard screw for 

 objectives (frontispiece). The stage should be quite large for the examina- 

 tion of slides with serial sections and other large objects. The substage fittings 

 should be so arranged as to enable one to use the condenser or to dispense en- 

 tirely with diaphragms. The condenser mounting should allow up and down 

 motion. 



\ 134. Quality and Cost. — In order that teachers and students may get 

 a good general idea of the appearance of microscopes of various makers for 

 high school and advanced laboratory work a few pictures are appended of the 

 microscopes most used in the United States. This has been rendered possible 

 by the courtesy of the manufacturers or importers. The microscopes are ar- 

 ranged in alphabetical order of the makers. 



Laboratory microscopes which will answer nearly all the requirements for 

 work in Biology, including Histology, Embryology, Pathology and Bacteri- 

 ology, are listed in the makers catalogs at about $75.00. The less expensive mi- 

 croscopes shown are listed at $25 to $45. Fortunately in the State of New York 

 the State pays half for high school apparatus, so that there is no reason why 

 every high school should not be properly equipped with microscopes of a good 

 grade. To avoid misunderstanding it should be added that the quality of the 

 oculars and objectives on the high school microscopes figured is the same as 



