288 ACCESSORY APPARATUS 



coloured glass as may be found needful, and the lamp illuminating 

 the paper and pencil, and carefully shaded above, is also seen at the 

 eye-piece end of the body-tube. Often, if the image is too bright, 

 we find that bringing the lamp down to illuminate the paper more 

 intensely suffices If not, use screens ; the illuminating cone must 

 not be tampered with. 



III. The Determination of Magnifying Power is an important 

 and independent branch of this subject. For this purpose, and for 

 the reason given above, Beale's neutral -tint camera l is eminently 

 suitable indeed, is the best. We can easily and accurately measure 

 the path of the ray from the paper to the eye. What is necessary is 

 to project the image of a stage micrometer on to an accurate scale 

 placed ten inches from the eye-lens of the eye-piece. There must be 

 complete accuracy in this matter. 



We can best show how absolute magnifying power is thus deter- 

 mined by an example. 



Suppose that the magnified image of two TTJ L^ths of an inch 

 divisions of the stage micrometer spans T 8 ^ths of an inch on a rule 

 placed as required ; then 



(i) '002 inch : '8 inch :: 1 inch : x power ; 



x-='-~ =400 diameters ; 

 002 



for it is obvious that under these conditions one inch bears the same 

 proportion to the magnifying power that T ^ ths of an inch bears to 

 i%-ths of an inch. 



Suppose, now, as it sometimes happens, that the operator is pro- 

 vided with a metrical stage micrometer, but is without a metrical 

 scale to compare it with, there being nothing but an ordinary foot- 

 rule at hand. 



Let it be assumed that the magnified image of two T ^ mm. when 

 projected covers T 8 F inch ; then, as there are 25'4 mm. in one inch, 



(ii) '02 mm. : (-8 inch x 25'4) :: 1 : x power ; 



8x25-4x1 1n1 ,. 



x= =1015 diameters. 



'02 



If the reverse is the case, viz. that you have an English stage 

 micrometer and a metrical scale, then, if the magnified image of two 

 1TT i__ths of an inch spans 18 mm., 



1 R 



(iii) -002 inch : "L : : 1 : x ; 

 .2o*4 



7087x1 o^ o A- 

 x= .= d54'o diameters. 



The above results indicate the combined magnifying power of the 

 objective and eye-piece taken at a distance of ten inches. The arbi- 

 trary distance of ten inches is selected as being the accommodation 

 distance for normal vision. 



The magnifying power, however, is very different in the case of 



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