8 THE AMERICAN MONTHLY [ January, 
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 0.60 and 0.30, however, are absolutely dependent on 
the compensating eye-pieces ; if used with the ordinary ones the images will 
be confused by color 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 vision 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 image remains quite colorless up to the 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 compared 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, figures for a rational numeration. 
On this basis the series of eye-pieces is ranged according to their magnifying 
power— 
I 2 4 8 12 18 247 
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 3.0 mm. focal 
length gives a magnification of 83.3 (at the conventional distance of 250 mm.) ; 
eye-piece 12, therefore, gives, with this objective, 12 < 83.3 = 1000, for the 
same distance of vision. 
In order to obtain the most favorable results it isnecessary that the eye-piece 
used on Continental and English microscopes, respectively, should be of dif- 
ferent formula, because of the very different paths which the rays take in the 
two cases, owing to the great difference in the lengths of the body-tubes. 
Both series are arranged to give precisely the same magnifying powers, the 
difference in the body-tube being compensated for by the focal lengths. 
The settings are so adjusted in both series that the lower focal point of all 
the eye-pieces lies at the same plane when inserted in the body-tube. No 
alteration of adjustment is, therefore, required on changing the eye-piece, 
and the optical tube-length (7. e., the distance between the upper focal point 
of the objective and the lower one of the eye-piece) , which is the standard fac- 
tor for the magnifying power, remains constant. This optical tube-length in 
the Continental microscope (excluding small differences between the various 
objectives) is equal to 180 mm., and in the English 270 mm., provided that 
the length of the body-tube, from the upper surface of the setting of the objec- 
tive to the upper end of the tube on which the eye-pieces rest, is 160 and 250 
mm., respectively. 
Compensating Eye-pieces. 
| Finder | F 5 
| eyespieces. | Working eye-pieces. 
Eye-piece magiaification ................ ali i ee eon pee Ney 
For|Continental tube. 
Equivalent focal length in mm.........| 180 | go | 45 liz2-5:| 225 || ton 
or|the English tube. 
rs Py i sa rsse7 S| 34s l22-5 aesee 
peace 
