32 MICROSCOPE AND ACCESSORIES. \_CH. I. 



Demonstrate this by screwing off the field-lens and using the eye-lens 

 alone as in the ocular, refocusing if necessary. Note also that the let- 

 ters or other image is bordered by a colored haze (§7). 



When looking into the ocular with the field-lens removed, the eye 

 should not be held so close to the ocular, as the eye-point is consider- 

 ably farther away than when the field-lens is in place. 



§ 55. The eye-point. — This is the point above the ocular or simple 

 microscope where the greatest number of emerging rays cross. Seen 

 in profile, it may be likened to the narrowest part of an hour glass. 

 Seen in section (Fig. 30), it is the smallest and brightest light circle 

 above the ocular. This is called the eye-point, for if the pupil of the 

 eye is placed at this level, it will receive the greatest number of rays 

 from the microscope, and consequently see the largest field. 



Demonstrate the eye-point by having in position an objective and 

 ocular as above (§ 49). Light the object brightly, focus the micro- 

 scope, shade the ocular, then hold some ground-glass or a piece of the 

 lens paper above the ocular and slowly raise and lower it until the 

 smallest circle of light is found. By using different oculars it will be 

 seen that the eye-point is nearer the eye-lens in high than in low ocu- 

 lars, that is the eye-point is nearer the eye-lens for an ocular of small 

 equivalent focus than for one of greater focal length. 



REFERENCES FOR CHAPTER I. 



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

 periodicals referred to. 



For the subjects considered in this chapter general works on the microscope 

 may be consulted with great advantage for different or more exhaustive treatment. 

 The most satisfactory work in English is Carpenter-Dalliuger. For the history of 

 the microscope, Mayall's Cantor Lectures on the microscope are very satisfactory. 

 See also B;ale, E Bausch, Beherens, Kossel and SchiefFerdecker, Dippel, Frev 

 Harting, Hogg, Nageli and Schweudener, Robin, Van Heurck, Clark, Cross and 

 Cole, Stokes. 



The following special articles in periodicals may be examined with advantage : 



Apochromatic Objectives, etc. Dippel in Zeit. wiss. Mikr., 1S86, p. 303 ; also in 

 the Jour. Roy. Micr. Soc, 1886, pp. 316, 849, mo ; same, 1890, p. 480 ; Zeit. f. Iu- 

 strumentenk., 1890, pp. 1-6; Micr. Built., 1891, pp. 6-7. 



Tube-lecgth, etc. Gage, Proc. Amer. Soc. Micrs., 1887, pp. 168-172 ; also in the 

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

 Proc. Amer. Soc. Micrs., 1890, pp. 43-49 ; also in the Microscope, 1890, pp'. 289-296^ 



Aperture. J. D. Cox, Presidential Address, Proc. Amer. Soc. Micrs., 1SS4 pp 

 5-39. Jour. Roy. Micr. Soc, i88[, pp. 303, 348, 365, 388 ; 18S2, pp. 300/460- isS*' 

 p. 790 ; 1884, p. 20. 



