852 SUMMARY OF CURRENT RESEARCHES RELATING TO 



employed is, by our own instrumentality, accessible to any one, and 

 no optician is in tbe least degree prevented from producing tbo same 

 objectives as good and as cheap as be can. 



Compensating Eye-pieces. — These new eye-pieces have been designed 

 for the purpose of compensating certain errors in the image formed 

 by the objective, outside the axis, which cannot be corrected in the 

 objectivo itself. They are specially arranged for use with the apo- 

 chromatic objectives, and materially improve their performance by 

 giving a uniformly colourless image. 



The eye-pieces may also be effectively used with relatively wide- 

 angled objectives of the old form, but when used with the ordinary 

 medium and low power dry objectives, the images which they give 

 outside the centre of the field are inferior to those obtained with the 

 eye-pieces hitherto used. On the other hand, the apochromatics of 

 0*95 and upwards allow of the use of ordinary eye-pieces without 

 any material detriment to their performance. The dry objectives of 

 0*00 and 0*30, however, are absolutely dependent on the compen- 

 sating eye-pieces ; if used with the ordinary ones the images will be 

 confused by colour-fringes. 



The compensating action of the eye-pieces on certain chromatic 

 aberrations in the objective-image, can be well seen with the higher 

 powers where the diaphragm limiting the field of view is outside the 

 lenses. The edge of this diaphragm will be found to show a deep 

 red border, whilst when used with the apochromatics the imago 

 remains quite colourless up to its margin. 



The classification of these eye-pieces is carried out on the principle 

 suggested by Prof. Abbe, viz. on the increase in the total magnifying 

 power of the Microscope obtained by means of the eye-piece as com- 

 pared with that given by the objective alone. The ratio of the 

 magnification obtained with an eye-piece and a given body-tube, to 

 the real magnification of the objective itself (or in other words, the 

 number which denotes how many times an eye-piece increases the 

 magnifying power of the objective, when used with such a body-tube) 

 gives the proper measure of the eye-piece magnification, and at the 

 same time the figures for a rational numeration. 



On this basis the series of eye-pieces is arranged according to 

 their magnifying power : — 



1 2 4 8 12 18 27 



the figures serving at the same time as the designation of the eye- 

 pieces. 



The magnification obtained by combining an eye-piece with any 

 objective, is arrived at directly by multiplying its number by the 

 magnifying power of the objective, as given in the preceding list. 

 An objective of 3 • mm. focal length, for example, gives a magnifi- 

 cation of 83 • 3 (at the conventional distance of 250 mm.) ; eye-piece 

 12, therefore, gives with this objective 12 X 83-3 = 1000 for the 

 same distance of vision. 



In order to obtain the most favourable results, it is necessary that 

 the eye-pieces used on Continental and English Microscopes respec- 



