786 
SUMMARY OF CURRENT RESEARCHES RELATINO TO 
A suggested Improvement in the Correction of Lenses for Photo- 
micrography.* — Dr. H. G. Piffard writes: — ‘‘Prior to the time of the 
late Colonel J. J. Woodward, M.D., Surgeon of the United States Army, 
say twenty-five years ago, photomicrography was in its first infancy. 
It is true that photographs of microscopic objects had been made, but 
they were crude and unsatisfactory, and were all made with what we 
would call low-power objectives. Although the objectives then made 
were of excellent construction and well adapted to the revelation of the 
structure of minute objects to the eye, yet the photographs made with 
them were greatly inferior in clearness and sharpness to the virtual 
image appreciated by the retina. The cause of this was not far to seek, 
and was due to the lack of coincidence of the visual with the so-called 
actinic focus. At the period mentioned the art of photography was 
almost exclusively practised with the aid of collodio-iodide plates, which 
were very sensitive to the blue, violet, and ultra-violet (more refran- 
gible), and but feebly sensitive to the green, yellow, and red (less re- 
frangible) rays. Per contra , these latter rays impress the eye so forcibly 
that the effect of the more refrangible rays is almost obscured — that is, 
when mingled with the others, as in ordinary white light. The practical 
outcome of this condition was, that when the ground glass of the camera 
was in a position that gave the sharpest image to the eye, this image 
could not be duplicated as to sharpness in the developed photographic 
plate occupying the plane previously occupied by the ground glass. 
In order to obtain a sharp photographic image it was necessary 
either to shorten the anterior conjugate focus, which involved the veriest 
guess-work, and was practically unavailable, or else to move the plate to 
a point nearer the lens where the actinic rays came to their posterior 
conjugate focus. This was perfectly practical, and by repeated experi- 
ment the relation of the actinic to the visual focus in a given lens 
could be ascertained. Although practical, this method was hardly 
satisfactory. 
In ordinary photography, the difficulty attending this difference in 
the natural positions of the actinic and visual foci had already been over- 
come by making the visual focus correspond with the actinic by con- 
structing the lens so that it should be left in a state of moderate 
‘ under-correction,’ as it is termed by opticians. 
Among the first to appreciate the value of this, as applied to photo- 
micrography, was Colonel Woodward, and the first opticians to give it 
practical form were, I believe, Mr. V/illiam Wales, of New York, and 
Mr. Ernst Gundlach, then of Berlin, but now for many years a practical 
optician in this country. This example was followed by Tolies, of 
Boston ; Powell and Lealand, of London, and others. 
Woodward was one of the most accomplished microscopists, so far as 
the manipulation of the instrument was concerned, that ever lived. His 
skill in securing the virtual image and in projecting the real image was 
at that time equalled by few and probably surpassed by none. 
With Woodward’s skill and the possession of lenses specially adapted 
to his purpose, the results obtained by him were the wonder of the 
scientific world. 
bince his time photomicrography has, in the main, followed the paths 
* Amer. Journ. Med. Soi., cvi. (1893) pp. 23-9. 
