CH. II. ] L A BORA TOR Y MICROSCOPES. 6 1 



LABORATORY COMPOUND MICROSCOPES. 



\ 114. Optical Parts.— A great deal of beginning work with the microscope in 

 biological laboratories is done with simple and inexpensive apparatus. Indeed if 

 one contemplates the large classes in the universities and medical schools, it can 

 be readily understood that microscopes costing from $25-50 each and magnifying 

 from 25 to 500 diameters, are all that can be expected. But for the purpose of 

 modern histological investigation and of advanced microscopical work in general, 

 a microscope should have something like the following character : Its optical out- 

 fit should comprise, (a) dry objectives of 50 mm. (2 in. ), 16-18 mm. (^ in. ) and 3 

 mm. (}i in.) equivalent focus. There should be present also a 2 mm. ( jV in.) or 

 1.5 mm. ( T \j in.) homogeneous immersion objective. Of oculars there should be 

 several of different power. An illuminator or substage condenser, and an Abbe 

 camera lucida are also necessities, 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. Cer- 

 tainly at the present, the apochromatics stand at the head, although the best achro- 

 matic objectives approach them very closely. 



\ 115. 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 ; 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 rapid move- 

 ment 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, and the draw-tube should be 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 exami- 

 nation of slides with serial sections. If there is no mechanical stage (§ 116), it is 

 also of considerable advantage to have the stage with a circular, revolving top, 

 and two centering screws with milled heads. In this way a mechanical stage with 

 limited motion is secured, and this is of the highest advantage in using powerful 

 objectives. The sub-stage fittings should be so arranged as to enable one to dis- 

 pense entirely with diaphragms, to use ordinary diaphragms, or to use the conden- 

 ser. The condenser mounting should allow up and down motion, preferably by 

 rack and pinion. The base should be sufficiently heavy and so arranged that the 

 microscope will be steady in all positions, and interfere the least possible amount 

 with the manipulation of the mirror and other sub-stage accessories. 



\ 116. Mechanical Stage. — There should also be present some form of mechani- 

 cal stage. That on the most expensive American and English microscopes for the 

 last twenty years and the one now present on the larger continental microscopes, 

 is excellent for high powers and preparations of moderate dimensions, but for the 

 study of serial sections and large sections or preparations in general, mechanical 

 stages like those shown in Figs. 68-69 are more useful. This form of mechanical 

 stage has the advantage of giving great lateral and forward and backward motion. 

 It is a modification of the mechanical stage of Tolles. The modification consists 



