32 Apertures of Ohject-glasses. 



of the illuminating pencil. From this fact it is evident that the 

 theoretical perfection of mounting objects in this manner would be 

 to enclose them exactly in the centre of a minute sphere of balsam. 

 In this case the pencil of rays, both from the achromatic condenser 

 and object-glass, would pass directly to the object without refraction 

 or diminution ; but I must confess that I have not been successful 

 in effecting this ; it is very easy to form a sphere of balsam at the 

 end of a needle point of any degree of minuteness, but very difficult 

 to coax an object into the centre of such a spherule." 



I have been engaged in countless comparisons of object-glasses 

 of different makers brought together by their respective owners, 

 and each held his private opinion of the result. I have avoided any 

 public mention of comparative merit as an assumed dictatorship 

 both odious and uncalled for ; and in this object-glass so confidingly 

 forwarded by Mr. Tolles, I would not sanction a trial against glasses 

 of any English manufacturer, as it was sent for a different purpose. 

 But as Mr. Tolles has stated in allusion to my Jth described in my 

 paper " On the Construction of Object-glasses " (published in the 

 commencing numbers of this Journal) as being composed of curves 

 that he would not use on any account, and that it would not give a 

 large angle in balsam (beyond that assigned by theory) because it 

 had " no collecting power" ! ! I did therefore venture to make the 

 comparison with this, of curves now obsolete, made twenty-two 

 years ago. It proved far superior to Mr. Tolles' (made three years 

 ago) on every object on which it was tested. I have sent my glass 

 to Dr. Lawson in order that he may also make the comparison. I 

 value it as a matter of history, as the first successful glass with a 

 triple back and single front, and showing Podura markings of a 

 light ruby tint on a green ground, then a novel appearance, but 

 now recognized as a criterion of perfect correction. 



I do not wish this to be taken invidiously, as Mr. Tolles may 

 have made a great advance since then, and may profit by the hint. 

 His glass appeared to have a compound front. The single front 

 may be used with advantage in all the powers from the \ upwards. 



I am now weary of urging these reiterated demonstrations both 

 theoretical and practical of the angles of immersed objectives, and must 

 drop the question, believing that my views are accepted by a discri- 

 minating majority, therefore no reply must be expected from me ; 

 Mr. Tolles' sect may set me down as an infidel having faith in what, 

 to them, is an unknown creed, or as a serpent deaf to the voice of the 

 charmer, but even this shall not provoke an answer. As I before 

 stated, I had no wish to discuss the question further with Mr. 

 Tolles, and the necessity of testing the aperture of his glass sent for 

 the purpose is an apology for my reappearance. 



F. II. Wenham. 



