442 MICROSCOPY. 
Mr. Wenham evidently does not recognize the possibility of 
“ collecting,” by means of posterior combinations, rays more di- 
vergent (behind the front lens) than those which are extreme 
when the objective is worked dry; and Mr. Tolles distinctly dis- 
avows this theory for his side of the controversy: yet it seems 
neither absurd nor improbable, and it is most likely the expedient 
by which the balsam angle is to be increased beyond 82°. 
Since the above was in type Dr. J. J. Woodward has published 
an important contribution on this subject in the Monthly Mic. 
our. A y was sent to him in February by Mr. Tolles for ex- 
amination. It gave good definition, through glass one seventy- 
fifth of an inch thick, at its point of highest cover-adjustment ; 
but at such adjustment its aperture could not be satisfactorily 
measured by the tank method. He therefore contrived an inge- 
nious modification of the card-board method, throwing parallel 
solar rays through the objective from above, and measuring, in a 
darkened room, the inverted cone of a light below the focus of 
the objective, by bisecting this cone of light with a thin flat tank 
filled with balsam or other medium, the objective being attached, 
immersion fashion, to the surface of the medium. The illuminated 
portion of the medium was easily seen and measured. This 
method gave a balsam angle of not over 80° to the 4, sent to him 
by Tolles for measurement, as well as to other Tolles’ lenses pre- 
viously furnished by that maker. On being apprised of this result 
Mr. Tolles sent a 1, which gave a balsam angle of 90° to 100°, 
according to adjustment. This objective was peculiarly con- 
structed, having four combinations instead of three; it could not 
be worked dry, nor could it work through any but a very thin 
cover. Dr. Woodward, and Prof. Simon Newcomb and Mr. Renel 
Keith, who examined the lens with him, attributed the excessive 
angle to the cause already alluded to, the employment of rays; 
which if the lens were worked dry would be beyond the limits of 
transmission, and would therefore suffer total reflection. 
Mountine rv Barsam.—Mr. W. H. Walmsley’s success in mount- 
ing objects gives great value to his practical suggestion contri- 
buted to Science Gossip. He regrets that beginners should be 
confronted with spring clips, spirit lamps, and over-heated balsam, 
when balsam, dried to the point of brittleness and then dissolved 
to the consistency of rich cream in chemically pure benzole, would 
obviate the necessity for such annoyances. He frees the speck 
