Phoio-micrography. 41 



DR. CURTIS'S PROCESS OF PHOTO-MICROGRAPHY. 



BY E. L. MADDOX, M.D. 



The beautiful specimens of photo -micrography, tracings of 

 which we were enabled, by the kindness of Dr. Maddox, to 

 lay before our readers in our July number, have excited so 

 much interest, that we are sure the following letter from Dr. 

 M. explaining the process will be highly appreciated : — 



" Through the kindness of Dr. J. J. Woodward, who has 

 charge of the Medical and Microscopical Department in the 

 Army Medical Museum, U.S., I am enabled to offer for the 

 pages of your valuable journal a brief description of the plan 

 adopted in taking the photo-micrographs alluded to, and pub- 

 lished in your July number, and which becomes of more in- 

 terest, as Dr. Woodward shows by a further number of prints 

 of various objects, taken both by sun and artificial light, the 

 excellency of the method. 



" The difference in sharpness given to the ^Vth over the -g-th, 

 and achromatic concave amplifier, is attributed by him to the 

 chemical process employed in rendering the albumen negative 

 intense. The smaller prints were from the original negatives, 

 and in the enlargement from these the solar camera was not 

 employed. 



"The diatom Pleurosigma angulatum was simply a dry 

 mounted specimen, adhering to the cover of an ordinary slide, 

 which was removed, turned, and then covered by the thin 

 glass necessary for the T Vth. The part alluded to as possibly 

 a sun spot, was only due to a particle of adherent dust. 



" The following plan is adopted by Dr. Curtis at the labora- 

 tory of the Museum — the lenses of the objectives in use being 

 corrected for the violet ray, and the formula furnished by Mr. 

 Lewis Rutherford, so well known for his astronomical photo- 

 graphy; but in the -^th, the difference between the visual and 

 actinic foci is so small as not to be noticed in the correction : — 



" Outside the window is placed a Silbermamr's Heliostat, 

 which reflects the rays from a plane mirror, duly centred, 

 through a brass tube, at the orifice of which hangs a large cell, 

 with parallel sides of plate glass, filled with a saturated solu- 

 tion of the ammonio sulphate of copper, the other arrange- 

 ments of the apparatus being properly disposed in a dark room, 

 instead of using a camera : thus practically violet monochro- 

 matic light is employed. The sensitized albumenized plates 

 being, in this case, placed about three feet from the object for 

 the -aVth, rather less for the -fth and amplifier, and seven 

 minutes' exposure allowed. The condenser used was a pair of 

 plano-convex lenses of about one inch combined focal length, 



