The Measuring Power of the Mi&roscojpe. By A. E. Wright. 281 



At the back of the micro-metrical scale there is mounted a 

 Eamsden eye-piece. This eye-piece serves for reading off the 

 dimensions of the image. It is manifestly open to us to employ 

 in this situation any magnifying power which may happen to be 

 convenient. In the instrument upon the table the eye-piece has 

 a magnifying power of 10 — that is to say, a magnifying power 

 which exactly balances the ten-fold minification before spoken of. 

 We obtain by this means, on placing the eye at the eye-lens of the 

 eikonometer, placed in position over the eyepiece of the Microscope, 

 an image of precisely the same dimensions as that obtained on look- 

 ing into the microscope in the ordinary way. On looking through 

 the eikonometer at a distant object we obtain in like manner 

 an image of the same dimensions as in ordinary unassisted vision. 



To complete the description, it may be pointed out that, in 

 addition to the focussing arrangement for the eye-piece, provision 



B 



Fig. 54. 



A, focussing lens, 1 in. focal length ; B, micronxetric scale (divided 

 into tenths of a millimetre) ; C, Ramsden eye-piece, magnifying 

 10 diameters ; D, stud working in spiral groove in E, outer sleeve" 



is 



made for bringing the focussing lens into the plane of 

 the Eamsden disc of the eye-lens. When this is done we obtain 

 in the eikonometer as extensive a field of view as in the Micro- 

 scope when the Eamsden disc of the eye-lens is, as is normally the 

 case, disposed in the pupil of the observing eye. 



In the arrangement adopted, the tube of the eikonometer moves 

 up and down in an outer sleeve — the movement being regulated 

 by a stud fitting into a spiral slot. 



In addition to measuring the magnifying power of the Micro- 

 scope the eikonometer will render services — 



(a) In the case where we desire directly to measure the magni- 

 fying power of the objective, or any other lens or combination of 

 lenses, which furnishes a system of pencils of parallel rays. 



(b) In the case where we desire to arrive at the magnifying 

 power of an optical element by subtracting from the cumulative 



