790 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Smith on the order of Dr. J. A. Fordyce, of this city, I have resolved 
the A. pellucida with white light, and have demonstrated the resolution 
to him and to others. With a 1/15 in. of the same construction, the 
property of Dr. J. H. Kellogg, of Battle Creek, Michigan, I made 
the photograph of the podura which accompanies this paper (fig. 113). 
In order to further test the correctness of the principles involved, I 
requested Mr. Turner, of the Gundlach Optical Co., Rochester, N.Y., 
to make a 3/4 in. lens of moderate aperture (N.A. 0*33). Considering 
the power, aperture, and price (12 dollars), the lens gave very satisfactory 
results, and wholly confirmatory of the theoretical demand. 
In using these lenses with yellow sensitive plates there is a distinct 
gain in definition and purity of image, both visual and photographic, if 
the object be illuminated with light of corresponding refrangibility 
(A. 5892). This may be obtained absolutely by employing a sodium 
flame, or approximately by intercepting the white light with a medium 
capable of absorbing the rays of short wave-length. 
For the greater part of photomicrographic work, and especially that 
which deals with histology, I unhesitatingly recommend the technique 
here described, which, briefly stated, consists in the use of objectives 
whose corrections shall be adjusted to the D instead of the G or H 
lines, and in connection with plates specially sensitive to D light, and 
having the object illuminated as near as may be with rays of the same 
refrangibility. 
If, however, we have to deal with objects in which we must resolve 
or optically separate particles whose approximation to each other is 
less than, say, 1/100,000 in., the foregoing statements do not apply. 
The studies of Helmholtz and of Abbe have placed us in possession 
of a formula w r hich appears to be theoretically and practically true, and 
may be expressed as follows : 
_ ^ n x sin u 
R. P. here indicates the resolving power of the objective ; n , the 
refractive index of the medium lying between the cover-glass of the 
object and the front lens of the objective (be the same air, water, 
glycerin, or oil) ; u, the semi- angle of the aperture of the objective ; 
sin, the natural sine of said semi-angle ; and A, the wave-length of 
the light employed. Now the £ numerical aperture * of the lens is equal 
to n x sin u, and the equation becomes simplified into 
E.P.= Ml. 
From this it will be seen that if we desire to obtain extreme resolu- 
tion, it is necessary to employ objectives of the greatest numerical 
aperture, and employ in connection therewith such visible rays as 
possess the shortest wave-lengths. For the photographic reproductions 
of such images blue-violet sensitive plates, with under-corrected lenses 
and approximately blue or violet illumination (Woodward’s technique) 
will give the best results. At the present time Mr. E. M. Nelson, of 
London, is devoting special attention to the development of this branch 
of photomicrography. 
