26 



MICROSCOPE AND ACCESSORIES. 



[CH. I. 



Fig. 35. Ocular Screw-Micrometer with 

 compensation ocular 6. The upper figure 

 shows a sectional view of the ocular and the 

 screw for moving the micrometer at the right. 

 At the left is shown a clamping screw to 

 fasten the ocular to the upper part of the mi- 

 croscope tube. Below is a face view, shozenng 

 the graduation on the wheel. An ocular 

 micrometer like this is in general like the 

 cob web micrometer and may be used for 

 measuring objects of varying sizes very accu- 

 rately. With the ordinary ocular microme- 

 ter very small objects frequently fill but a part 

 of an interval of the micrometer, but with this 

 the movable cross lines traverse the object {or 

 rather its real image) regardless of the minute- 

 ness of the object. (Zeiss' Catalog, No. jo). 



39. Spectral or Spectroscopic Ocular. — (See Micro-Spectroscope, Ch. VI). 



DESIGNATION OF OCULARS. 



\ 40. Equivalent Focus. — As with objectives, so ne opticians designate the ocu- 

 lars by their equivalent focus (I 13). With this method the power of the ocular, 

 as with objectives, other lenses or lens systems, varies inversely as the equivalent 

 focal length, and therefore the greater the equivalent focal length the less the 

 magnification. This seems as desirable a mode for oculars as for objectives and is 

 coming more and more into use by the most progressive opticians. It is the 

 method of designation advocated by Dr. R. H. Ward for many years, and was 

 recommended by the committee of the American Microscopical Society, (Proc. 

 Araer. Micr. Soc, 1S83, p. 175, 1SS4, p. 228). 



^4t. Numbering and Lettering. — Oculars like objectives may be numbered or 

 lettered arbitrarily. When, so designated, the smaller the number, or the earlier 

 the letter in the alphabet, the lower the power of the ocular. 



\ 42. Magnification. — The compensating oculars are marked with the amount 

 they magnify the real image. Thus an ocular marked X 4, indicates that the real 

 image of the objective is multiplied four fold by the ocular. 



The projection oculars are designated simply by the amount they multiply the 

 real image of the objective. Thus for the short or 160 mm. tube-length they are, 

 X 2, X 4 ; and for the long or 250 mm. tube, they are X 3 and X 6. That is, the 

 final image on the screen or the ground glass of the photographic camera will be 

 2, 3, 4, or 6 times greater than it would be if no ocular were used. See Ch. VIII. 



COMPOUND MICROSCOPE. 



EXPERIMENTS. 



§ 43. Putting an Objective in Position and Removing it.— Ele- 

 vate the tube of the microscope by means of the coarse adjustment 

 (frontispiece J so that there may be plenty of room between its lower 



