232 



MICRO-PROJECTION WITH OCULARS 



ICH. IX 



358. Micrometer ocular for demonstration. It is so difficult 

 for most students to understand the workings of the ocular micro- 

 meter, that it is of great help to them to use a micrometer ocular 

 like fig. 130 to 131 on the projection microscope, then the object 

 and micrometer lines can be projected together by suitably adjust- 

 ing the eye-lens of the ocular. A stage micrometer might also be 

 used as object and the students shown, all together, how to deter- 

 mine the ocular micrometer valuation (see Gage, The Microscope). 



Oculir lo 2 



FIG. 129. COMPENSATION OCULARS. 



(From Zeiss' Catalogue, No. jo). 



A section has been removed to show the construction. 

 The numbers 2, 4, 8, /2, 18, indicate the magnification of each 

 ocular (see 357a, 391 a). 



359. Substage condensers. The writers believe, from their 

 experience and experiments in photometry under the different 

 conditions, that it is better to use for illumination only the large 

 condenser (fig. 121). 



The use of a substage condenser is for either one of two purposes : 

 (i) to enable the position of the object and the projection objective 



The average increase in magnification given by the different oculars with the 

 different objectives and screen distances shown in the table ( 377) is as follows: 

 Projection ocular X2 gives a magnification of ................... 1.99 



*4 " ................... 3-^9 



Compensation " X2 " ................... 2.05 



Huygenian x4 " ................... 4.2 1 



From these figures it is seen that the increase in magnification for projection 

 work can be closely enough approximated by multiplying the image given by 

 the objective alone by the number designating the ocular, i. e., 2 or 4. 



If very precise results are desired, one must use a stage micrometer and pro- 

 ceed as described in 391 a. 



