9 
and marginal portions of the field do, therefore, not appear in sharp focus 
simultaneously, but have to be focussed in succession. 
As a result of the great concentration of light afforded by these objectives, 
they permit of the use o f v e r y high power e y e - p i e c e s without detriment 
to accuracy or brightness of the image, thus giving high magnifying power with 
relatively long focal length, tlius yielding with the same objective a series of 
very varying amplifications. 
In the annexed list beside the apertures and foci the corresponding 
objective magnification is stated, i. e. the magnification which the ob- 
jective alone would give at the distance of distinct vision if used as a simple 
lens. This is simply 250 (distance for distinct vision) divided by 
the focal length of the objective in mm. For instance, the 
objective magnification of a 3-mm objective is: 
The apertures indicated are tlie guaranteed minimum values; the 
stated focal lengths are adhered to as closely as possible. 
Remarks on the use of tlie 2-mm Apocliromatic Lens of 1.40 mm aperture 
(homogeneous immersion). 
Owing both to the hyper-hemispherical form of the front lens, which is held 
in situ by a very narrow ridge at the extreme edge of the setting, and its rela- 
tively short working distance (0.2 mm) this lens demands careful treatment 
and should, in particular, be scrupulously guarded against shocks or pressure. 
As compared with the 2-mm lens of 1.30 aperture this lens possesses a greater 
resolving and defining power in the proportion of 14 : 13 and gives a brighter 
image in the proportion of 20 : 17. The 2-mm 1.40 is a decided advantage where 
exceptionally intricate problems requiring the most delicate apparatus 
have to be solved, and also in such cases where the lens is to serve as a means 
of controlling observations made with lower powers; for regular work 
preference should be given, generally speaking, to the 2-nim 1.30, owing, to the 
greater firmness of the frontal lens. 
The objective 3-mm 1.40 apert. has, however, this advantage over the 2-mm 
1.40 that, while possessing the same optical capacity, it has a greater working 
distance and is also less delicate as regards its mechanical construction. 
