64 LA BORA TOR Y MICROSCOPES [ CH. II 



LABORATORY AND HIGH-SCHOOL COMPOUND 

 MICROSCOPES 



\ 121. 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 high schools, the universities and med- 

 ical 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 outfit should comprise, (a) dry objectives of 50 mm. (2 in.), 16-18 mm. 

 (%in.)and 3 mm. {)/% in.) equivalent focus. There should be present also a 

 2 mm. ( T : 2 in. ) or 1.5 mm. (y 1 ^ in.) homogeneous immersion objective. Of oculars 

 there should be several of different power. A centering 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 

 achromatic objectives approach them very closely. 



\ \ii. Mechanical Parts or Stand. — The stand should be low enongh 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 hori- 

 zontal. The adjustments for focusing should be two, — a coarse adjustment or 

 rapid movement with rack and pinion, and a fine adjustment by means of a mi- 

 crometer 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 examination 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 entirely with diaphragms. The condenser mount- 

 ing should allow up and down motion. 



STANDARD SIZES RECOMMENDED BY THE ROYAL 

 MICROSCOPICAL SOCIETY 



I 123. Society Screw. — Owing to the lack of uniformity in screws for micro- 

 scope objectives, the Royal Microscopical Society of London, in 1857, made an 

 earnest effort to introduce a standard size. The specifications of this standard are 



