28 MICROSCOPE AND ACCESSORIES. 



in.), i6-i8mtn. {${ in.) and 3 mm. (% in.) equivalent focus. There should be present 

 also a 2 mm. ( T x j in.) or 1.5 mm. (y 1 ^ in.) homogeneous immersion objective. Of 

 oculars there should be several of different power. An Abbe illuminator, and an 

 Abbe camera lucida are also necessities. A micro-spectroscope and a micro-polar- 

 izer 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. 



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 movement 

 with rack and pinion, and a fine adjustment by means of a micrometer screw. 

 Both adjustments should move the entire body of the microscope. The body or 

 tube should be short enough for objectives corrected for the short or 160 millime- 

 ter tube-length, and the draw-tube should be graduated in centimeters and milli- 

 meters. The lower end of the draw-tube and of the tube should each possess a 

 standard screw for objectives (PI. II, Fig. 10). The stage should be quite large 

 for the examination of slides with serial sections ; it is also of considerable advan- 

 tage 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 se- 

 cured, and this is of the highest advantage in using powerful objectives. The 

 sub-stage fittings should be so arranged as to enable one to dispense entirely with 

 diaphragms, to use ordinary diaphragms, or to use the Abbe illuminator. The il- 

 luminator mounting should allow up and down motion, preferably by rack and 

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

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

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



A microscope with all the accessories mentioned above would cost about $170 

 in Europe. Without the micro-spectroscope and micro-polarizer about $115. 

 To get the American price of a foreign instrument or the catalogue price of one 

 of American manufacture, the amount of duty (i. e., 60 per cent.) should be added. 



REFERENCES FOR CHAPTER I. 



In the appendix will be given a bibliography, with full titles, of the works 

 and periodicals referred to. In the text the works are mostly designated by single 

 letters, and the periodicals by the initial letters of their titles. 



The works of Beale (B.), Bausch (Bau.), Behrens (Beh.), Carpenter (C), Dippel 

 (D. ), Frey (F.), Hogg (H.), Mayall, Nageli und Schwendener, Robin. 



The following special articles in periodicals ma}* be examined with advantage : 



\ 14. Apochromatic Objectives, etc. Dippel in Zeit. wiss. Mikr. 18S6, p. 303 ; 

 also in the Jour. Roy. Micr. Soc, 18S6, pp. 316, 849, 1110 ; same, 1890, p. 480 ; Zeit. 

 f. Instrumentenk., 1890, pp. 1-6 ; Micr. Built., 1S91, pp. 6-7. 



I 17. Tube-length, etc. Gage, Proc. Amer. Soc. Micrs., 1S87, pp. 168-172 ; also 

 in the Microscope, the Jour. Roy. Micr. Soc, and in Zeit. wiss. Mikr., 18S7-8, 

 Bausch, Proc. Amer. Soc. Micrs., 1S90, pp. 43-49 ; also in the Microscope, 1890, 

 pp. 289-296. 



\ 18. Aperture. J. D. Cox, Presidential Address, Proc. Amer. Soc. Micrs., 1884, 

 PP- 5^39- Jour. Roy. Micr. Soc, 1881, pp. 303, 348, 365, 38S ; 1882, pp. 300, 460; 

 18S3, p. 790 ; 1S84, p. 20. 



§ 54. The Abbe Illuminator. Archiv f. mikr. Auat., Vol. IX, (1S73) P- 4 6 9- 



