26 



MICROSCOPE AND ACCESSORIES. 



[CH. I. 



Eig. 35- Ocular Screw-Micrometer with 



compensation ocular 6. The upper Jig lire 

 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 lube. Below is a face view, showing 

 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. Willi the ordinary ocular microme- 

 ter very smalt objects frequently fill but a part 

 of an interval of the micrometer, but with this 

 the movable cross lines traverse the object (or 

 rathcrits real image) regardless of the minute- 

 ness of the object. (Zeiss' Catalog, No. 30). 



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



DESIGNATION OF OCULARS. 



jj 40. Equivalent Focus. — As with objectives, so. ne opticians designate tbe ocu- 

 lars by their equivalent focus [\ 13). With this method tbe power of tbe ocular, 

 &i with objectives, other leases or lens systems, varies inversely as tbe equivalent 

 focal length, ami therefore the greater the equivalent focal length tbe less tbe 

 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 tbe American Alicroscopical Society, ( Proc. 

 Amer. Micr. Soc, 1883, p. 175, 1884, p. 22,8). 



'i 41. Numbering and Lettering. — Oculars like objectives may be numbered or 

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

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



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

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

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



Tbe projection oculars are designated simply by tbe amount they multiply tbe 



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



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



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



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



COMPOUND MICROSCOPE. 



EXPKRIMF.NTS. 



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

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

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



