28 PEACTICAl, TREATISE ON 



length of both be equal, say/' says Sir D. Brewster,* « one- 

 eightieth of an inch, the magnifying power of the diamond lens 

 wiU be 2133 diameters, whereas that of glass would be only 

 800." Mr. Pritchard, in later times, succeeded, with much 

 less difficulty, in making lenses of other precious stones, viz., 

 the sapphire, ruby, and garnet, all these substances, although 

 coloured to a certain extent, nevertheless were not unfitted 

 for magnifying powers; and Sir David Brewster, whose 

 authority is indisputable in these matters, states : f — " That 

 they all exhibit minute objects with admirable accuracy and 

 precision, and that the colour of the garnet, which diminishes 

 with its thickness, disappears almost whoUy in very minute 

 lenses." The durabihty of lenses made of the diamond and 

 other precious stones, is, however, an exceedingly valuable 

 property; but the vast expense incurred in their manufacture, 

 and the great superiority of the compound instrument, as now 

 constructed, wHl ever be a barrier to their introduction into 

 general use. 



The microscope, with a single lens, having been brought 

 to the greatest state" of perfection by the labours of Sir David 

 Brewster, Dr. Goring, and Mr. Pritchard, we must here leave 

 it, and direct our attention to certain combinations of lenses 

 termed doublets and triplets, by means of which microscopic 

 science has been considerably advanced, and, with the ex- 

 ception of the achromatic compound microscope, no more 

 important improvement in the optical part of the microscope 

 has ever yet been accomphshed. As long ago as the year 1688, 

 a doublet was described in the Philosophical Transactions, as 

 made by Eustachio Divini,| in which a large and flat field was 

 obtained by placing two planoconvex lenses so as to touch 

 each other in the middle of their convex siirface. "This 

 instrument," it is there stated, "hath this peculiar, that it 

 shews the objects flat and not crooked, and although it takes 

 in much, yet nevertheless magnifieth extraordinarily." In the 

 year 1812, a periscopic doublet lens was proposed by Dr. 

 WoUaston, II it was composed of two planoconvex lenses, 



* Treatise on the Microscope, p. 21. f Op. Cit, p. 24. 



JNo. 42, p. 842. \\ Philosophical Transactions, 1812, p. 375. 



