397 
XL — On a Method of Adjusting the Sizes of the Coloured Images 
yielded by the Coolce Lens . 
By Philip E. Bertrand Jourdain, F.RM.S. 
( Read 18 th May , 1898.) 
The problems involved in designing objectives for photomicrography 
in three-colour printing include not only the bringing together to one 
focus the rays from three different points in the spectrum, so as to 
correct the outstanding secondary spectrum, but the securing of the 
same sized image for the same object in three different colours. 
The former problem can, of course, be practically solved by the 
use of special glasses, such as those used by Schott & Co. ; the second 
part of the problem was first explained in detail in the writings of 
Dr. Schroder. 
One of the most interesting points about the Cooke lenses, which 
Mr. E. M. Nelson kindly showed on my behalf before this Society 
on March 16th last, is their capability of being adjusted so that the 
sizes of the coloured images correspond— -a capability which is quite 
exceptional, and has probably been attained in only one other objective, 
the “ Concentric ” of Dr. Schroder. 
In order to understand the process of adjustment, it is necessary 
to have a clear idea of the construction of the lens. It is a triplet, 
composed of two simple positive lenses and a simple or compound 
negative lens * placed between them : the negative lens or element 
having a refractive index for the D ray lower than that of the positive 
lenses. The focal power of the negative lens falls slightly short of 
the sum of the powers of the two positive lenses, and a small residue 
of correction against curvature of field and astigmatism is performed 
by diaphragm corrections.f 
In securing coincidence of the coloured images with such an 
objective, the best results can be practically obtained by the careful 
adjustment of the air-space between the back and negative lenses, and 
this end may be attained without sacrificing any other qualities of 
the lens. 
In practice, this adjustment is obtained by treating the photo- 
graphic or microscopic objective as the objective of a telescope, using 
as the ocular a high power and carefully corrected achromatic eye- 
piece. 
* In the lenses sliown the negative lens was simple and the aperture F/6’5. 
t If the refractive indices of the three lenses are about equal, strict application of 
the principle of construction demands that diaphragm corrections be totally elimi- 
nated, so that the burden of correcting curvature of field and marginal astigmatism 
be thrown on the negative lens, whose power would have to be equal to the combined 
focal powers of the positive lenses. As this would necessitate thick glasses and deep 
curves, which tend to produce aberrations of secondary order, the best final results 
are obtained by the compromise described above. 
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