86 il/r. Tolles^ " Experiments on Angular Aperture." 



a, Fig. 2, is a tank with a plate-glass bottom filled with water,* 

 h is the light some distance below on the floor; the sector for 



measuring the apertures is set ver- 

 f'G-2 tically, so that the nose of the 



object-glass c, may remain immersed 

 in the water during the rotation. 

 The following are some results : — 

 A T^Vth of 170° (immersion front) 

 showed an apparent aperture of 

 100°; a |th of 130° = 87°, and a 

 T%th of 90°= 67°. If a higher 

 refractive medium than water had 

 been employed, the angles would 

 have been still less, and with Canada 

 balsam, or something of the same 

 refractive power of the glass, the 

 aperture from 170° could not exceed 

 80°. This experiment would have 

 been more inconvenient to try, and 

 I did not think it worth while to 

 run the risk of injuring my object- 

 glasses for demonstrating a simple 

 fact in known optical laws quite 

 incontrovertible. About 80° is the 

 utmost aperture that we can expect 

 to obtain for an object mounted in 

 balsam ; and the principle does not 

 differ, whether we employ an immersed front or not. The latter, 

 of course, has the advantage in transmitting more light, and allows 

 greater control over the aberrations. A tank might be used with 

 a glass side, and the sector kept horizontal as usual, but this 

 requiring an elastic water-tight connection from the object-glass, 

 would be troublesome. 



Messrs. Tolles and Stodder have girded on a convex front, and 

 then ventured forth to make a stand against my unwelcome state- 

 ments — that the aperture of object-glasses is reduced on balsam- 

 mounted objects, and that there is no subsequent increase of this 

 aperture by using an immersion lens. I had not the pleasure of 

 shaking hands with them before the collision, but in the absence 

 of this ceremony, I hope they may take the reception that they 

 have met with in good part. 



* The glass-bottomed metal pot, used by fastidious beer-drinkers, will answer 

 the purpose, but I do not know whether our Transatlantic friends are familiar 

 with this. 



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'4i. 



mm 



