288 ACCESSORY APPARATUS 



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

 the paper and pencil, and carefully shaded al>ove, is also seen at the 

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

 \\r find that bringing the lamp down to illuminate the paper more 

 inten>ely suilices I f not, use screens ; the illuminating cone must 

 not ! tampered with. 



TIT. The Determination of Magnifying Power is an important 

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

 t In- reason given above. Beale's neutral-tint camera 1 is eminently 

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

 ili,. p.-itli of the ray from the paper to the eye. What is necessary i* 

 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 l>y an example. 



Suppose that the magnified image of two r ,-/,-,- jjths of an inch 

 division* of the stage micrometer spans njths of an inch on a rule 

 placed as required ; then 



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



'8x1 



,' ' =400 diameters; 

 002 



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

 proportion to the magnifying power that , , MH1 ths of an inch bears to 

 j^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 ] s ll inch ; then, as there are 25'4 mm. in one inch. 



(ii) -02 mm. : (-8 inch x 2.V4) :: 1 : x power; 



8 x 2.r4 x 1 



10 ](} diameters. 



'()_ 



[fthe reverse is the ca.se. vix. that you have an English stage 

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

 ,,,',,,, ths of an inch spans 18 mm., 



(iii) -002 inch :_*.?_:: \ :X ; 



7087x 1 

 x= o =354'3 diameters. 



Theahoxe n-Milt.s indicate the combined magnifying power of the 

 ol>jecti\e and eye piece taken at a distance often inches. The arbi- 

 trary distance of ten inches is selected as heing the accommoda.tion 

 distance for normal \ i.sion. 



'lie ma--iiifviiig power, bowever, i.s verj dilfei-ent in the case of 



i p a 



