828 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



resj)ectively, and by p'/2 tlie distance of the latter point from the nearest 

 refracting surface, we have 



But the formula of conjugate foci gives 



<i^- f-d-f,-f' 



Thus p'/2 is known since p'^ is given by experiment, and so also 

 r]'^ since 



^2 = /2 - P'/a- 



In the following table are given the results of applying the preceding 

 experimental methods and calculations to a Verick Microscope with 

 objective No. 2 and eye-piece No. 1. 



Experimental Data. 



p'f^ = _ 2"™-87 

 pep = 2"'"'- 78 



p'<t, = 15"'"' -15 



where $„ $2 denote the dioptric power of the whole Microscope with 

 eye-piece in place and drawn out through 62'""' -5 respectively, J9<|) the 

 distance of the first focal point of the whole Microscope from the first 

 face, and p'fi the distance of the second focal point of the objective from 

 its last face. 



CaUulated Results. 



1"'"'-15 pf^ = 23"'"' -85 



l"'°'-63 q'^ = 13'""' -918 



lQmm.37 y^ ^ l'°'"-232 



14'""' -87 7]'^ = -42'""' -246 



- 67"^"" -33 



where. q<p denotes the distance of the first focal point of the whole 

 Microscope to the first focal point of the _ objective, and p/, the distance 

 of the first focal point of the objective to its first face. 



The author concludes by pointing out the advantages which would 

 result if constructors of Microscopes would take care to provide the 

 micrometer-screws with a graduation, and would furnish with every 

 instrument the optical constants which alone determine its scientific 



value. 



The author arranges his conclusions under the following twelve 



heads: — . , . , .^ • .r. 



I. The magnification of an optical instrument does not give the 



measure of its useful effect. 



II. The amplifying power is equal to the product of the magnification 

 by the ratio of the distances of the eye to the object and to its image. 



III. Two kinds of amplifying power may be distinguished: the 

 absolute and the relative. For the Microscope the latter is the more 

 important. 



IV. The relative amjjlifying power is equal to the dioptric power 



