Process for making White Lead. 253 



excellent optician Mr. Ramsden, by the combination in the 

 best position of two piano glasses, with their convex sides 

 to each other, applied eye-pieces to his instruments with 

 great advantage, to read off divisions of his circles, and 

 iTiaenifv the wires of his telescopes with clear definition at 

 the circumference of the field of view, the diameters of the 

 glasses being no larger than the aperture of ihe tube. The 

 same principle has since been advantageously applied to 

 large object lenses for the lucernal microscope, by the late 

 Mr. G. Adams and ourselves, where the diminution of 

 light was of less consequence than indistinctness of the 

 images. In many cases the combination of two convex 

 lenses answers well ; but the combining of two similar 

 plano-convex lenses together, of superfluous diameter and 

 thickness, and for the greatest defect or aberration, in the 

 worst position to each other, and afterwards to palliate it 

 with a small aperture, as shown in fig. 4, (Plate IV.) is 

 such an anomaly or absurdity in optics as not to require 

 any serious coninKnt on my part. I shall only appeal to 

 tile least experienced constructor of microscopes, whether 

 he does not know that the substitution of a double convex 

 lens, of the diameter only of Dr. Wollaston's aperture, and 

 of the same focus, would produce an image infinitely more 

 perfect and vivid than the mutilated lens proposed by Dr. 

 Wollaston. 



From these remarks, I presume, there will be nothing to 

 apprehend from the attempt of Dr. Wollaston to depreciate 

 the excellence of the spectacles, camera obscuras, and mi- 

 croscojies, which have been constructed by themost eminent 

 opticians of the day. 



Yours, &c. 



Holhorn, April 13, 1813. WiLLIAM JONES. 



XXXIX. il/. Montgolfirr's Process for making White 

 Lead. By Messrs. Clemust ajid D esq rmes*. 



JL HK celebrated Montgolficr's process for the manufacture 

 of white lead, being one of great simplicity, ought to he 

 better known; and with ih.s view we have drawn up the 

 following description. 



The first operation consists in forming the lead into 

 .sheets. He found from experience, that by running the 

 melted metal on ticking, the slieets might be made of any 

 thinnc8d, and v.iricd at will, by inclining the frame a little 



* /tuialts de Chimie, No. 240. 



