146 
The A'pjplication of 
cases. For the purpose of soaking out such stains, I myself very 
much prefer to use a strong solution of caustic potash, as described 
by Yirchow in 1857.* This reagent dissolves the fibrin and sets 
the red blood-corpuscles free without materially modifying them. 
In order to obtain photographs of the specimens thus prepared, 
which shall at once be well defined and magnified sufficiently 
to render diff'erential measurements easy, it is advisable to use 
immersion objectives of high power. I have employed the 
Powell and Lealand xeth, and the y^th of Tolles, belonging to 
the Army Medical Museum, for this purpose, and, rejecting all 
eye -pieces and amplifiers, have aimed to obtain a magnification 
not less than 1000 diameters by distance alone. Of the two 
objectives named, the one by Tolles gives somewhat the sharper 
images, and has the flatter field. I have therefore used it for 
most of the work. It is only just to repeat here that, although 
correctly named a Tsth, this objective is of lower power than 
the wet front of the Powell and Lealand yVth, belonging to the 
Museum, which is really a y 9th, although the dry front of the same 
combination is a yVth. The chief difficulty in making photographs 
of blood-corpuscles with this high power is to avoid diffraction 
fringes in the images ; but this can readily be done by following the 
method which I have explained in detail in my paper on " Photo- 
graphing Histological Preparations by Sunlight." t This method 
is extremely simple, and offers no difficulties to anyone who has 
acquired a reasonable degree of skill in microscopical manipulation. 
It is my desire to put it at the service of all sincere workers in this 
direction, and I cannot therefore view with indifference the pub- 
lication of imperfect substitutes, which can only serve to waste the 
time of anyone who may be misled into employing them. For 
this reason I feel it a duty to warn the reader against attempting 
to follow the methods described in two recent papers on this 
subject, published in the ' Philadelphia Medical Times.' % It would 
be waste of time to criticise these papers in detail. The methods 
described in them appear to have been devised without any intel- 
ligent consideration of the optical principles involved. I was not 
therefore surprised, on seeing some of their author's photographs of 
blood-corpuscles with high powers, referred to in his second paper, 
to find them lacking in definition, full of diffraction fringes, and, by 
reason of these faults, unfit for measurement or any other serious 
* R. Virchow, " Ueber die forensische Untersuchung von trockenen Blut- 
flecken," Virchow's ' Archiv,' bd. xii. s. 334. 
t Surgeon-General's Office, July 13, 1871. Eeprinted in the ' American 
Journal of Science and Arts,' October 1871, p. 258, the ' Monthly Microscopical 
Journal,' October 1871, p. 169, and the British Journal of Photography, 
October 27, 1871, p. 507. 
X Carl Seller, " Photographic Enlargements of Microscopical Objects," 'Phila- 
delphia Medical Times,' June 5, 1875, p. 5G3. By the same: "High Powers in 
Micro-photogiaphy," op. cit., February 10, 1875, p. 249. 
