An Apparatus for Obtaining the " Balsam" Angle. 213 



i. e. for uncovered objects, 145°. By the new method hinted of 

 above, the balsam angle is 77°, less by 2° than Mr. Wenham's 

 trial made it. 



At closest, to the extent of one-half of its whole adjustment, I 

 now get 90° in balsam instead of 93° as before. These discrepancies 

 are, I am confident, chargeable to the tank method which I before 

 used, and which I used because it was not my method. 



I now use a semi-cylinder of crown glass, reading the angle on 

 the cylindrical surface, where the rays emerge. Of course the 

 cylindrical surface has a non-polished portion to facilitate the true 

 reading. I will send a description of the apparatus, but not in 

 time to appear with this note. 



Boston, March 19, 1873. 



I conclude to send a sketch of the apparatus I have used to get 

 the real balsam angle actually available in use of whatever objective. 



The candle-flame F (Plate XV., lower portion) shows the 

 position in which the apparatus is seen in the figure. 



B and E, the base of the apparatus. B is a sector of 180°. 

 A is a semi-cylinder of crown glass closely to the refractive index 

 of balsam, and having an elevation above the base B, beyond the 

 axis of the tube and objective D. A film of covering glass is 

 represented at C. 



As the light from the candle-flame F proceeds down the tube 

 and emerges at the front of the objective, and traverses the balsam- 

 mounted object at C, it will have divergence and reach the cylin- 

 drical surface of the glass halt-cylinder, emerging without sensible 

 refraction. 



By means of a proper shutter swinging over the surface-cylin- 

 drical of A, the light can be admitted from the outside of it, and 

 by means of shutter the exact limits of angle be found, while the 

 observer has the object in view through the eye-piece and objective. 

 That this can be utilized as a condenser, with peculiar advantages, 

 is evident enough. 



The rude drawing will answer present purposes, but I hope to 

 communicate explicit results hereafter. But I desire to state here 

 that the results given in my table of measurements of balsam angle 

 are substantially verified by this method of measure. 



40, Hanover Street, Room 30, Boston, 

 March 21, 1873. 



P.S. — The author adds, " the drawing is now to scale of ■ 65 

 to 1 in.," giving at the same time permission for its reduction. 

 This permission has been accepted. The sketch is now but frds of 

 its original size as sent by Mr. Tolles. 



