242 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
to the prism-casing. The screw P serves to adjust the scale with respect 
to the spectrum. 
The instrument is inserted in the tube in place of the ordinary eye- 
piece, and is clamped by the screw M in such a position that the mirrors 
A and 0, which respectively serve to illuminate the comparison-prism 
and the scale, are simultaneously illuminated by sunlight. 
Determination of the Focal Length of Objectives.* — M. P. Fran- 
cotte regrets that opticians do not mark their objectives with their focal 
lengths instead of by letters or by arbitrary numbers. He finds that 
with a tube-length of 160 mm., using the compensating eye-piece 4 of 
Zeiss, the value of one division of a micrometer eye-piece in which 1 mm. 
is divided into 100 parts, is equal in microns to the number which 
expresses the focal length of any objective used. For any other eye- 
piece the same result follows by varying the tube-length. Thus, if the 
length of the tube be fixed once for all for any eye -piece, in order to 
find the focal length of an objective, it is sufficient to determine with 
the aid of the objective and an objective micrometer the value of one 
division of the eye-piece micrometer ; the number of /x obtained is then 
the same as that which expresses in millimetres the focal length 
sought. 
Eye-piece with Iris-Diaphragm.f — Hr. 0. Zaeharias calls attention 
to a new eye-piece recently brought out by Zeiss, which has proved very 
serviceable in biological work.f 
* Bull. Soc. Beige de Microscopie, xxi. (1894-5) pp. 208-15. 
t Biol. Centralbl., xvi. (1896) pp. 30-1. 
t See this Journal, ante , p. 120. 
