On Improvements of the Microscope. By Prof. E. Ahhe. 31 



medium eye-pieces now employed. The diameter of the eye-lens is con- 

 siderable, and the eye-point is so far from the lens that the camera lucida 

 may he used without any difficulty. 



Thirdly, the endeavour to realize as far as possible the advantage of an 

 objective of relatively greater focal length, which was before indicated, 

 has given occasion to extend the series of eye-pieces downwards also, 

 below the present limits, so that for one and the same objective a very 

 large variation may be given to the magnifying power. For this purpose 

 two special eye-pieces of unusually long focal length have been made, 

 the weakest of which produces an eye-piece magnifying power = 1 with 

 the Continental tube, i, e. which produces with every objective exactly that 

 magnifying power ivhich would he given hy the objective used as a lens 

 without any eye-piece. These eye-pieces may appropriately be termed 

 ^'Searcher eye-pieces,'' because they are adapted not so much for 

 systematic observation as for a preHminary view of and search over the 

 object. Including these searcher eye-pieces, the whole series gives to 

 each objective a range of useful magnifying power varying from 1 to 18, 

 so that for example, a homogeneous-immersion objective of 3 mm. focal 

 length, which gives a magnifying power of 1500 with the strongest 

 eye-piece, when used with the weakest yields only the small magnifying 

 power of about 80. 



In the above discussion frequent use has been made of a method of 

 characterizing the eye-piece by the ratio of the total magnifying power 

 of the Microscope to that of the objective. The author has elsewhere 

 established this method for determining the action of the eye-piece in 

 the Microscope.* Attention may, however, here be called to the practical 

 advantage of the method as a basis for a rational designation of eye- 

 pieces. 



If the combination of objective and eye-piece in a Microscope 

 produces a linear magnifying power N, referred to the conventional 

 image-distance I, this expresses the fact that under these conditions 

 the Microscope as a whole forms a lens-system of focal length 



f = -' Then for every system of lenses, whatever its construction, 



N = -^ , and thus a definite value of N results hy virtue of a definite 



value of/. To discover what proportion of the magnifying power of the 

 whole Microscope belongs to the eye-piece, it is necessary to compare 

 the instrument as it is with the eye-piece with what it woiild be without 

 the eye-piece. In the latter case it is a system of lenses (the objective) 

 with any other greater focal length F, The ratio between this focal 

 length and that of the whole Microscope is therefore a numerical ex- 

 pression of the eye-piece action in the Microscope. This ratio is 



F F,, N 



7 = T^ = 7r 



9' 



v.F. 



See this Journal, 1883, p. 791 et seq. 



