164 HOW TO SEE WITH THE MICROSCOPE. 



is written with singular tact and perspicuity, and is here 

 reprinted unchanged : 



A /Simple Devwefor the Illumination of Balsam-mounted 

 Objects for Examination with certain, Immersion Ob- 

 jectives whose "Balsam Angle" is 90 or upwards. 

 By Surgeon J. J. WOODWAKD, Brevet Lieut. Col. U. 

 S. Army. 



[Taken as read before the ROYAL MICROSCOPICAL SOCIETY, 



June 6, 1877.] 



" Certain immersion objectives are so constructed that 

 they are capable of admitting rays which enter the front 

 lens at a greater angle with the optical axis than the 

 limit for dry objectives. That this is not only theoretic- 

 ally possible, but that such objectives have been success- 

 fully constructed, was several years since demonstrated 

 in the " Monthly Microscopical Journal" both by Mr. 

 Keith and myself,* notwithstanding which the contrary 

 lias often since been energetically asserted by writers in 

 the same Journal. 



" Meanwhile, immersion lenses possessed of the exces- 

 sive angle in dispute, continue to be put into the market 

 by more than one maker; and perhaps some of the pur- 

 chasers will be interested in a simple device which I have 

 used for some time with such objectives to illuminate 

 test-objects mounted in balsam. This device consists 

 merely of a right-angled prism of crown glass mounted 

 beneath the stag-e in such a manner that its lon^ side 



o o 



can be connected by oil of cloves, or some similar fluid, 



*June, 1873, p. 268; November, 1873, p. 210. March, 1874, p. 119; September. 

 1876, p. 134. 



