420 Dr. Goring on Achromatic Microscopes* 



in the way I have discovered, and described in my paper on; 

 the Amician microscope. Another means may be also re- 

 commended, which is to measure with the same instru- 

 ment the size of the image of some distant body, such 

 as a window, &c., formed in the visual pencils, taking 

 care that the instruments are at the same distance from the 

 object when the experiment is made. As to ascertaining 

 power by any means which takes into consideration any 

 standard of sight, it will be always subject to disputes, be- 

 cause every man has a standard of his own ; and therefore, on 

 this principle, it may be asserted that the magnifying power 

 of a microscope is in fact different to every different observer. 

 I myself generally examine a minute microscopic object at 

 about four and a-half inches distance from my eye ; another 

 cannot see it nearer than eight or ten inches : so it may be said 

 that the microscope magnifies as much again to him as it does 

 to me. On this account it would be much more convenient 

 for observers to mention the focus of the single lenses they use 

 instead of their powers ; and, if they use a compound instru- 

 ment, to state the focus of a single lens to which its power is 

 equal, according to the method first employed by myself, and 

 which has been already sufficiently described. Not only, then, 

 are the powers to be equalized, but, in compound instru- 

 ments, the length of the body and the focus of the object- 

 glass or metal must be also the same ; for the penetrating 

 power ceteris paribus perpetually increases along with the 

 depth of the objective part. This is an eternal argument for 

 object-glasses and metals of short foci^ used with shallow eye- 

 pieces. I shall thus explain my meaning : — suppose I try 

 three instruments against each other, viz. a common compound 

 microscope, having an object-glass of ^^^^th of an inch focus and 

 ^^o^h of an inch aperture, and such a body and eye-piece at- 

 tached to it as shall raise its power to 240 (or that of a lens 

 of ^^yth of an inch focus). I have also an achromatic in- 

 strument, with an object-glass of 0*4 focus and 0*2 aperture 

 raised to the same power of amplification by an adequate eye- 

 piece or an increased length of tube ; together with an Ami- 

 cian reflector, having an objective metal of 0*9 focus and 045 

 aperture also raised to the same power : let us suppose, for ar- 



