32 Transactions of the Society. 



and since here ^^ denotes the magnifying power n which would be 



obtained from the objective alone, it follows that the total magnifying 

 power of the Microscope should always be compared with the 2^'^ojper 

 magnifying power of the objective, although the latter is not generally 

 made use of, since IMicroscope objectives are not also used as lenses. 



The division of the action of the whole Microscope between objective 

 and eye-piece is therefore shown by the formula 



N = w I/; 

 F 



F 



where v denotes the quotient — , the measure of the action of the eye- 

 piece. 



N 



As the author has shown, the ratio v - — determines at the 



n 



same time the whole influence which is exercised by tube and eye-piece 

 upon the character of the microscopic image. If in any Microscope 

 the total magnifying power N exceeds the magnifying power n of the 

 objective, say tenfold — as for example, if with an objective of 5 mm. focal 

 length, of which the magnifying power determined in the usual way 

 is only 50, the total magnifying power is brought to 500 — then this 

 number z; = 10 denotes the whole optical conditions on which the 

 character of the image under these circumstances depends. In par- 

 ticular, it is made clear that all faults of image formation which originate 

 in the objective will be magnified exactly tenfold in the final image, 

 whether this tenfold enlargement is produced by a long tube and weak 

 eye-piece, or the reverse. 



Now in practice, it is of considerable interest to be always re- 

 minded of this important factor in the use of the Microscope, and this 

 can be accomplished if the designation of the difierent eye-pieces ex- 

 presses directly the s«j3<?r-magnifying power which they produce. There 

 is also here the further advantage that the total magnifying power is at 

 once obtained by multiplying the number of the eye -piece by the proper 

 magnifying power of the olrjective, which is given by its focal length. 



This designation of the eye-pieces is adopted with the new con- 

 structions. Each eye-piece is denoted by a number representing the 

 super-magnifying power which it gives to the I\Iicroscope with the 

 normal length of tube for which it is constructed. Having regard to 

 the prevalent confusion in the designation of eye-pieces, it would be 

 a great advantage if opticians in general would adopt this method of 

 rational notation. 



In this system, of course an eye-piece can only have a definite 



magnifying-power number so far as it is used with a definite length 



of tube, because the action of an eye -piece depends not upon its focal 



length alone, but also upon its distance from the objective. The ratio 



N 



— = V employed above is always determined by the value of the quotient 



n 



—, where <f> denotes the focal length of the eye-piece, and A is the dis- 

 



