170 On an Improved Method of 



rings, produced by diffraction and interference, which disturb the 

 normal appearances of the preparation and render its interpretation 

 impossible. 



If the image be received upon a white screen similar phenomena 

 obtrude themselves, destroying the clearness of the picture, though 

 no longer injuriously affecting the eye ; and if monochromatic hght 

 is employed, although the disorderly play of colour disappears, black 

 rings and lines of the most manifold character and direction take 

 their place. Pictures produced under these circumstances are of 

 course quite useless, and the difficulty occurs not merely in the case 

 of tissue preparations, but in a very large number of other objects. 



To escape these disagreeable results it has heretofore been the 

 practice to pass the solar pencil through a piece of ground glass. 

 This plan is recommended in all the treatises on photo-micrography, 

 and has hitherto been employed in the solar work done at the Army 

 Medical Museum. The method is effectual in getting rid of the 

 diffraction and interference phenomena complained of ; an image is 

 obtained which is clear and satisfactory to the eye looking down the 

 tube, but it appears very weak on the screen and is sadly deficient 

 in contrast. These faults are reproduced in photographs of objects 

 thus illuminated, and, moreover, the time of exposure is enormously 

 increased. Such pictures are decidedly inferior to those which can 

 be obtained by the Magnesium, or even by the Calcium light, with 

 which no ground glass is used. 



I desire now to call your attention to the fact that in the course 

 of some recent experiments I have ascertained that the diffraction 

 and interference phenomena above complained of, may be prevented 

 by the use of a suitable condensing lens, even better than by the 

 ground glass; that by this plan the exposure may be greatly 

 diminished, say from three minutes for five hundred diameters, to a 

 fraction of a second, and that the resulting pictures are not merely 

 quite as free from diffraction and interference phenomena as the 

 best that can be obtained when the ground glass is used, but are 

 characterized by greater contrast and superior sharpness of defi- 

 nition. 



The details of my new method are as follows : — The microscope 

 being placed on a shelf at the window of the dark room, and its 

 body made horizontal, the achromatic condenser is illuminated by a 

 solar pencil reflected from a heliostat upon a movable mirror outside 

 the shutter and thence into the dark room, precisely as described in 

 my original paper on photo-micrography.* No ground glass is 

 used, but instead a lens mounted in a suitable tube is fixed in the 

 opening of the shutter through which the solar pencil enters. This 

 lens is an achromatic combination about two inches in transverse 



* ' American Journal of .Science and Arts,' September, 1866. 



