Chap, i.] Refleffing felefcope, &c. 167 



quently that the whole beam cannot be brought to a 

 focus in any one point, fo that the focus of every ob- 

 jed-glafs will be a circular fpace of confiderable dia- 

 meter, namely, about one fifty- fifth of the aperture of 

 the telefcope. To remedy this, he adopted Gregory's 

 idea of a reflector, with fuch improvements as have 

 been the bafis of all the prefent inftruments of this 

 kind. 



When a fcience has been carried to a certain degree 

 of perfection, fubfequent difcoveries are too apt to be 

 confidered as of little importance. The real philofo- 

 pher will not, however, regard the difcoveries on light 

 and colours, fmce the time of Newton, as unworthy his, 

 attention. By a mere accident, a very extraordinary 

 property in fome bodies of imbibing light, and after- 

 wards emitting it in the dark, was obferved. A Ihoe- 

 maker of Bolognia, being in quefl of fome chemical 

 fecret, calcined, among other things, fome Hones of a 

 particular kind, which he found at the bottom of 

 Mount Peterus, and cafually obferved, tha" when thefe 

 {tones were carried into a dark place after having been 

 cxpofed to the light, they poffefled a felf-illuminating 

 power. Accident afterwards difcovered the fame pro- 

 perty in other fubftances. Baldwin of Mifhia, diflblv- 

 ing chalk in aqua-fortis, found that the refiduum, after 

 diftillation, exactly refembled the Bolognian ftone in 

 retaining and emitting light, whence it now has the 

 name of Baldwin's phofphorus j and M. Du Fay ob- 

 ferved the fame property in all fubftances that could 

 be reduced to a calx by burning only, or after folution 

 in nitrous acid. Thefe fe&s feem to eftablifh the ma- 

 teriality of light. 



Some very accurate calculations were made about 



the year 1725 by Dr. Bradley, which afforded a more 



M 4 convincing 



