30 Apertures of Object-glasses. 



ceremony of sending a glass across the Atlantic for trial, I expected, 

 in this particular instance, to find some light beyond the theoreti- 

 cal angle, and hoped to investigate and explain the cause in order 

 to furnish some incidental proof of fallacy in the measurement of 

 extreme apertures ; but the limits of the immersed angles in this 

 case were more decided than in the air, and there was nothing 

 beyond the mere record left for me to do. Dr. Curtis has given his 

 faith to the trial without proof that he has paid any attention to 

 the principles of refraction involved in the experiment, and as Mr. 

 Tolles has shown that he will not be led by any theory, and there- 

 fore is quite unprejudiced thereby, I can only infer that he has 

 conducted the experiment in some way without regard to the laws 

 that should guide the rays through the media, and that a false 

 indication has resulted. 



It needs but a very limited knowledge of optical theory to 

 demonstrate that the utmost angle of possible transmission or 

 conversely of emergence from the first surface of ordinary crown 

 glass with a refractive index of 1*531 does not exceed 40° 43', and 

 therefore with still lighter crown, the angle behind the first surface 

 cannot get beyond 82°; and supposing the other lenses to be of 

 such a form as to bring to a posterior conjugate focus the rays of 

 even such an improbable angle, this cannot be increased either with 

 water or balsam immersion, without destroying that focus and giving 

 a negative result with no image in the eye-piece ; therefore this 82° 

 must continue straight in the balsam medium if of a similar refrac- 

 tion to the glass. 



I demonstrated this seventeen years ago, and till now no one 

 has disputed the position and at the same time tried the plans for 

 obtaining full aperture both of object-glass and illuminator, that 

 are now revived as new facts. I make the following extract from 

 the ' Quart. Journal of Mic. Science,' No. XII., July 1855, by which 

 it will be seen that having succeeded with the hemispherical lenses, I 

 felt somewhat diffident about encysting objects in Mr. Tolles' 

 "Pilluloe." 



"The sharpness and beauty with which some test-objects are 

 displayed under the diminished aperture consequent upon balsam 

 mounting, is, on first consideration, rather surprising, and tends to 

 show analogically the very great increase of distinctness that would 

 be obtained if the object could be seen in the same medium, with 

 the full aperture of the object-glass. Having been rather curious 

 to know if objects in balsam could be observed under such an ad- 

 vantage, I have tried a few experiments which were successful in 

 their results. 



" I first took a small hemispherical lens of about ^jth. of an inch 

 radius, and cemented it over a selected specimen of one of the 

 Diatomacese (N. Sigma) with Canada balsam, in the manner 



