1882.] in the object-glasses of telescopes. 231 



by the way, as far as I can judge, is of quite average goodness, 

 though decidedly green, as most old glasses are. 



I may be allowed to say, without pretending to give an opinion 

 of any value on a subject to which I have never given otherwise 

 than superficial attention, that while M. Cauchoix deserves credit 

 for a new and valuable invention, in the simple use of rock-crystal 

 instead of crown-glass, he seems to me to be mistaken in his idea 

 that the focal length or draw of telescopes ought to be shortened 

 thereby. If the generally known theories on these subjects are true 

 I should have said that the focal length of an object-glass 

 ought practically to depend on its diameter; and I think this is 

 generally the case; though I believe opticians generally make them 

 as short as they well can. For example, of two telescopes, one an 

 old one of English make of 2f inches object-glass, I find the pro- 

 portion 1 to 15^ ; of a newer French one, of Si inches diameter, 

 the proportion is 1 to 139. A common day telescope I possess, 

 not new, is 1 to 13|, i.e. not considering the erecting lenses. And 

 if the simple and easy rules given by Sir J. Herschel in the Edin- 

 burgh New Philosophical Magazine for 1821 are correct, it is 

 assumed that an achromatic object-glass may be made of any given 

 focal length, by varying the radii of the surfaces of the lenses 

 according to the dispersive ratio of the glasses employed ; but the 

 focal length of the compound lens is assumed to be always the 

 same, though I should add that the proper diameter of the object- 

 glass is not discussed by him. It is true that the table given there, 

 p. 367, curiously enough will not be available when we use rock- 

 crystal instead of crown-glass. Most likely, as I have said before, 

 at- that date (1821) the aim of inventors was to make flint-glass of 

 much higher specific gravity, and so of much higher dispersive 

 power. In the Bakerian Lecture for 1829 Faraday describes the 

 experiments by which he, in conjunction with a committee of the 

 Royal Society, had made a flint-glass of specific gravity 5 '3, em- 

 ploying oxide of lead ; and it must be in reference to this that it is 

 said in the Astron. Nadir., vol. VII., in the year 1829, that he had 

 made an object-glass of 2| inches diameter with a flint-glass of that 

 specific gravity, which had succeeded very well ; but I am sorry to 

 say that want of knowledge prevents me giving any farther in- 

 formation as to what was done in that direction. Still it would 

 not be difficult to calculate the radii of the surfaces on Herschel's 

 rules ; and I apprehend that this is what my optician has done. It 

 will only be necessary to extend Herschel's table, which now ex- 

 tends from "50 up to 75 dispersive ratio down to '40, at most. 



I am inclined therefore to think that M. Cauchoix's idea that 

 he could shorten the focal length of object-glasses by using quartz, 

 was not founded on theory, but only on some such practical ex- 

 perience as leads opticians to shorten telescopes when used I'.u 



