[CH. I 



MICROSCOPE AND ACCESSORIES 



29 



micrometer. The micrometer may be removable, or it nay be permanently in 

 connection with the ocular, and arranged with spring and screw, by which it 

 may be moved back and forth across the field. (See Ch. IV.) 



\ 49. Spectral or Spectroscopic Ocular. (See Micro-Spectroscope, Ch. 



VI.)' 



DESIGNATION OF OCULARS 



\ 50. Equivalent Focus. As with objectives, some opticians designate 

 the oculars by their equivalent focus (2 15 ). With this method the power of 



No. 2 



FIG. 38. Projection Oculars with section 

 removed to show the construction. Below are 

 shown the upper ends with graduated circle to 

 indicate the amount of rotation found necessary 

 to focus the diaphragm on the screen. No. 2, 

 No. 4. The numbers indicate the amount the 

 ocular magnifies the image formed by the 

 objective as with the compensation oculars. 

 (Zeiss* Catalog.) 



the ocular, as with objectives, varies inversely as the equivalent focal length, 

 and therefore the greater the equivalent focal length the less the magnifica- 

 tion. This seems as desirable a mode for oculars as for objectives and is com- 

 ing 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. 

 Amer. Micr. Soc., 1883, p. 175, 1884, p. 228). 



51. Numbering and Lettering. Oculars like objectives may be num- 

 bered 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. 



\ 52. Magnification. The compensation oculars and the Huygenian ocu- 

 lars of some makers are marked with the amount they magnify the real image. 

 Thus oculars marked X 4, X 8, indicate that the real image of the objective is 

 magnified four or eight 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, X2, X4 ; and for the long or 250 mm. tube, they are X3 an d X6. 

 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. 



