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, 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. With the ordinary ocular micrometer 

 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}. 



\ 44. 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. 



$ 45. 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 magnified 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, 

 X2, X4 ; and for the long or 250 mm. tube, they are X3 and 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. 

 46. Standard Size Oculars. The Royal Microscopical Society of London 

 took a very important step (Dec. 20, 1899) in establishing standard sizes for ocu- 

 lars and sub-stage condensers. To quote from the Journal of the Royal Micro- 

 scopical Society for 1900, p. 147 : 



Resolved, "That the standard size for the inside diameter of the substage fit- 

 ting be 1.527 in. =38. 786 mm. That the gauges for standardizing eye-pieces be 

 the internal diameters of the draw-tubes, the tightness of the fit being left to the 

 discretion of the manufacturers. ' ' 



The sizes for oculars are four in number, I and 2 being most common. 

 ( i ) 0.9173 inch = 23.300 mm. This is the Continental size. 



This is the size used by the English Opticians 



for student and small microscopes. 

 Medium size binoculars (English.) 

 Long tube binoculars. 

 For the history of the Huygenian Ocular, and a discussion of formulae for its 

 construction, see Nelson, J. R. M. S., 1900, p. 162-169. 



EXPERIMENTS 



47. Putting an Objective in Position and Removing it. 

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



(2) 1.04 inch = 26.416 mm. 



(3) 

 (4) 



1.27 

 1.41 



inch =32.258 mm. 

 = 35.814 mm. 



