1886.] 3Ir. H, Gruhh on Telescopic Objectives and Mirrors. 413 



WEEKLY EVENING MEETING, 



Friday, April 2, 1886. 



William Huggins, Esq. D.C.L. LL.D. F.R.S. Vice-President, 

 in the Chair. 



Howard Grubb, Esq. F.R.S. F.R.A.S. 



Telescopic Objectives and Mirrors : their Preparation and Testing. 



It would probably lend an additional interest to a technical subject 

 such as I have to bring before you to-night, could I preface my de- 

 scription of the processes now employed in the construction of tele- 

 scopic objectives by a short historical account of what has been 

 attempted and achieved in the past, but time will not permit. 



A very few words, however, on the history of glass manufacture are 

 necessary. 



As I pointed out last Saturday afternoon, Dollond's brilliant 

 discovery, which afforded a means of achromatising objectives, ren- 

 dered possible their construction of greater size and perfection than 

 formerly, provided suitable material could be obtained. But the 

 chromatic errors being removed, faults in the material hitherto 

 masked by them, were detected, and it was not until after many 

 years that Guinand, a lowly but gifted Swiss peasant, succeeded in 

 producing glass discs of a considerable size and free from these 

 defects. 



The secrets of his process have been handed down in his own 

 family to Mens. Foil, of Paris (one of his descendants), and also 

 through M. Bontemps, who for a time was associated with Guinand's 

 son, and afterwards accepted an invitation from Messrs. Chance Bros. 

 & Co., of Birmingham, to assist them in an endeavour to improve 

 that branch of their manufacture. Only these two houses, so far as I 

 am aware, have succeeded in manufacturing optical discs of large 

 size. 



Testing op Optical Glass. 



Let me here say a few words respecting the testing of optical 

 glass ;. I mean of the material of the glass, quite apart from the 

 optician's work in forming it into an objective. When received from 

 the glass manufacturer it is sometimes in this state, roughly polished 

 on both sides, and sometimes in this, in which as you see there are 

 small windows only, facets as they are called, polished on the edges. 

 In case of lenses for telescopic objectives, it is always well to have 



