564 Mr potter, ON A NEW CORRECTION, &c. 



expression =0, when the values of /, and t, are given for any par- 

 ticular lens. I have performed the calculation to this effect for the 

 above example, supposing the last surface of the flint-lens only to be 

 altered, and I find that its radius {r,) must be increased by ^'^ of an 

 inch nearly, which is a very small quantity in 14 feet, and shews us 

 that it is scarcely to be hoped, that we can obtain a very fine object- 

 o-lass by trusting to theoretical computations solely ; but that, after the 

 general forms have been investigated for the optician, we must rely 

 on his experience to vary his curvatures slowly until he has obtained 

 the maximum effect of distinctness. 



With respect to the actual thicknesses adopted in England, I am 

 indebted for information to the liberality of Mr Tully and Mr Robin- 

 son ; and as it is important that such information should be recorded 

 in print, I shall not hesitate to give the full extract from Mr Robin- 

 son's reply to my letter to him requesting such information. He says : 

 "Thinking that the best information might be obtained from Mr 

 Tully, I called on him, and not being so fortunate as to find him 

 at home, I left your letter, with the request that he would furnish 

 the information you required; he has just now called on me and tells 

 me, that there is no absolute rule for the thickness of either the 

 concave or convex lens; that great thickness for the convex lens, if it 

 be of crown glass, is considered objectionable on account of its colour 

 occasioning loss of light; and its being thin is objected to, but merely 

 because if it be ground to a sharp edge, there is danger of the edge 

 beino- broken in polishing: he has just made an object-glass of 5^ 

 inches diameter, the thickness of the crown glass is at the edge \ of 

 an inch, and that of the concave |, and these he thinks very proper 

 thicknesses and would not wish they should be thicker; but had these 

 disks been thinner, they, (being of good glass,) would not have been re- 

 jected on that account; and in general, the only rule for thickness is, 

 that it be such that the edge of the convex be not splintered in 

 working, and the centre of the concave be not so thin as to change 

 its form in polishing." 



