266 Transactions of the 



II. — Note accom]panying Three Photographs of Begeeria domes- 

 tica, as seen with Mr. Wenham's Black-ground Illumination 

 and a Power of 1000 diameters. By Dr. J. J. Woodward, 

 U. S. Army. 



{Read before the Eotal Microscopical Society, Nov. 1, 1871.) 



Mr. Wenham having very kindly sent me one of the small trun- 

 cated lenses designed by him to obtain, under certain conditions, 

 black-ground illumination with high powers, I carefully tried it on 

 the Begeeria domestica in the manner described in his paper in the 

 July number of the ' Monthly Microscopical Journal,* using the 

 immersion ^^th of Powell and Lealand as the objective. I had 

 no difficulty whatever in obtaining the appearances of the scale 

 described by Mr. Wenham and also several other aspects, and must 

 regard the contrivance as a valuable addition to our means of 

 studying semi-transparent objects with high powers. 



The illumination of the scale by this method, when a coal-oil 

 lamp was the source of Hght, was so brilliant that I thought pro- 

 bably it would be possible to photograph some of the more striking 

 appearances. I obtained on the first trial three negatives, of 

 which I send prints. The same scale is shown in each magnified 

 1000 diameters. The objective (the immersion iVth) remained at 

 the same cover correction, the position of the truncated lens and 

 parabola was unaltered, and the different appearances exhibited 

 resulted from trifling alterations in the position of the plane mirror 

 by which the parallel solar pencil was thrown upon the parabola, 

 and shght modifications of the fine adjustment. Of the prints sent. 

 No. 2 agrees pretty well with Mr. Wenham's description. The 

 same can hardly be said of No. 3, however, and Nos. 1 and 3 are 

 only examples of some of the manifold results attaiaable by this 

 method with which, indeed, almost as many appearances can be 

 seen as with transmitted light. 



The time of exposure required for these negatives was but three 

 minutes. I infer from this and many other circumstances, that the 

 semi-transparent scales are simply made luminous by the fight 

 passing into them from below, whence it results that the surface 

 appearances are necessarily complicated by the optical properties of 

 the structures beneath. 



I understand this to be substantially Mr. Wenham's view 

 also, and am therefore at some loss to comprehend the sense in 

 which he speaks of the scales as being shown " opaquely " by this 

 method.t 



* P. 7. t P. 7, and note loc. clt. 



