12 
In order to predetermine by calculation the aniplitications obtainable from 
these lenses, the distance of the image (i. e. of the focussing plate) from the ob- 
jective has to be divided by its focal length, i. e. by 70 and 35 respectively and 
the quotient so obtained is to be diminished by unity, in accordance with the 
well known formula. 
N = ^-l. 
Prices : 
Projection lens of 35 mm focus M. 35.— 
ad 2. The two newly constructed apochromatic objectives of relatively short 
focus present the same principal features as all the older types of this class of 
lenses. In particular, they require the use of "Compensating oculars", with this 
exception, however, that — in consequence of the relatively greater initial ampli- 
fication of the new systems — the highest numbers of eye-pieces should not be 
used with them, it being in the case of histological and bacteriological investigations 
not even advisable to go beyond the No. 8 eye-i)iece. 
ad 3. In autumn 1880 we constructed a system which sensibly exceeds 
with regard to defining p o w e r the limits which had existed for the then known 
objectives whose apertures were of necessity numerically less than the refractive 
indices of crown-glasses. The objective has an aperture of 1.60 to 1.63 with a 
focus of 2.5 mm and may satisfactorily be used with the ordinary compensating 
eye-pieces. The slight traces of colour which are observable near the margin of 
the field may, if thought necessary, be neutralised by adapting a correcting lens 
to the eye-pieces No. 8, 12 and 18. Pure monobromide of naphthaline serves as 
the immersion-fiuid to this objective, and a small bottle of it is supplied with 
each objective. The specimens have to be covered with carefully polished flint 
cover-glasses having an exactly calibrated thickness (of ab. 0.17 mm) and must 
be embedded in media of a refractivity exceeding 1.60 (e. g. monobromide of 
naphthaline, realgar, iodide of mercury, metliylene-iodide. Smith's medium etc.). 
When oblique illumination or an illuminating cone having an aperture of 
more than 1.40 is to be employed, flint slips, a flint-glass condenser and some 
highly refracting substance connecting both — say monobromide of naphthaline — 
have to be used. 
To obviate misconceptions regarding the capabilities of this lens we will not 
omit to distinctly state that we do not proifer this lens as an improved means of 
microscopical research directly applicable to the adopted micro-technical methods : 
