400 
Transactions of the Society. 
Zeiss projection lens of the same focus. The “ Planar ” lenses are 
quite achromatic, and the chromatic differences of spherical aber- 
ration are only very slight. The secondary spectrum is not, however, 
eliminated to the same extent as it is in the projection lens, and 
Dr. Rudolph does not think that this will ever be the case. It may 
be noted, however, that apochromatism is not a great necessity in 
low-power objectives. 
The objectives of 20 and 35 mm. focus may be used with Zeiss’ 
compensating eye-piece No. 18, but with such high magnifications a 
red border is visible.* 
* Further information as to curves and kinds of glass used -will be found in the 
patent specification (No. 27,635 of 1896), price 8 d., and its reprint in the Brit. Journ. 
Phot., xliv. (1897) pp. 421-6. 
