( 29 ) 



VII. — Apertures 'of Object-glasses. By F. H. Wenham. 



On the 14th day of November, 1872, the dry and immersed aper- 

 tures of Mr. Tolles' T V th were tested in presence of the undersigned. 



The angle in air (taken at the best adjustment for a Podura 

 scale) measured 145°. 



With the front in water, the angle became reduced to 91°. And 

 lastly in balsam the result was 79°. 



Chas. Brooke, F.R.S., V.P.R.M.S. 

 H. Lawson, M.D., F.R.M.S. 

 W. J. Gray, M.D., F.R.M.S. 



S. J. M'Intire, Esq., F.R.M.S. 



For measuring the immersed apertures the sector was fixed 

 vertically. The object-glass was set with its surface level to the 

 centre of the arc. A small circular tank was firmly clamped 

 below, into which the end of the object-glass passed. A wax 

 candle was placed exactly in a line on the floor three feet beneath. 



After the air aperture had been taken, the tank was filled with 

 water so as to cover the end of the object-glass. At each extreme 

 of the aperture a pencil line was drawn along the straight edge of 

 the .,bar that carried the microscope body, leaving a permanent 

 record of the trial. The water was then turned out and replaced 

 by fluid balsam, and the diminished aperture again marked with 

 pencil, the object-glass having been radiated in the fluid as before. 



A trial in the presence of such competent judges might well end 

 the question, and establish the infallible law by which such a result 

 could be foretold. 



From the result of the water trial I asserted that there would be 

 a further reduction of 15° in balsam or an aperture of 76°, but 79° 

 was indicated. These extra three degrees beyond my theoretical 

 limit may be accounted for in this way. I had assumed the index 

 of refraction for hard balsam. That used was very fluid, and there- 

 fore contained turpentine, which of course diminished the refractive 

 power. 



Dr. Josiah Curtis has witnessed an experiment conducted by 

 Mr. Tolles, and speaking in a somewhat jubilant tone of his 

 assumed triumph, remarks, " Equally gratified probably will Mr. 

 Wenham be when he shall see for himself that an angle of more 

 than 82° can be attained through balsam." * 



I may say that I certainly was surprised at seeing a result so 

 strictly in accordance with my argument; for after all this 



* See ' M. M. J.,' Nov. 1872, p. 243. 



