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XII . — Remarks on the Construction of the Planar Lens and its 
TJse in Low-Power Photomicrography. 
By Philip E. Bertrand Jourdain, F.R.M.S. 
{Read 18 th May , 1898.) 
The application of short-focus photographic lenses to low-power 
photomicrographic work has not always been satisfactory, though 
it is probably true that a short-focus photographic lens is preferable 
to a low-power microscopic objective for purposes of photography.* 
Recently, Dr. Rudolph computed a photographic lens of extremely 
large aperture, having regard to its astigmatic and spherical correc- 
tions, which, in the smaller sizes, is admirably adapted for low-power 
photomicrography. 
Fig. 74. 
In construction, the objective is symmetrical ; each half consisting 
of a positive single element L b and a negative cemented combina- 
tion, which, in turn, is made up of a positive biconvex L 2 cemented 
to a negative biconcave L 3 . The refractive indices of L 2 and L 3 are 
approximately equal, but L 3 has the higher dispersive power. 
Spherical aberration is corrected with a large aperture by taking 
advantage of the principle of Gauss’ telescope objective ; chromatic 
aberration is compensated in an entirely new manner ; and astigmatism 
is corrected without the disadvantages of a large separation between 
the lenses or thickness of the elements. 
For photographic work, it is chiefly interesting on account of its 
great available rapidity and wonderfully perfect astigmatic correc- 
tion ; indeed, in this respect it surpasses the other anastigmats calcu- 
lated by Dr. Rudolph and introduced by Carl Zeiss. 
For photomicrographic work, it is interesting to compare it with a 
* See Brit. Jonm. Phot., xliv. (1897) pp. 70G-7. 
