26 On the Eigh-Fower Definition of Minute Organic Particles. 



borate of lead glass placed between crown and flint with cement has 

 advantages in the correction of spherical and chromatic aberration 

 hitherto unattainable. It is to be hoped that at no distant date 

 great improvements will be effected. 



Mr. Harcourt's experiments carried forward for forty years for 

 the British Association in the construction of optical glass by 

 means of a gas furnace. 



The following substances were experimented upon, formed into 

 glasses chiefly with phosphates on account of the pasty character 

 of the silicates, combined in many cases with borates, tungstates, 

 molybdates, or titanates. The glasses formed involved the 

 elements of — 



Mr. Vernon Harcourt made nearly 166 prisms. Titanic acid 

 was found to have a superior power in extending the blue end of 

 the spectrum, whilst boracic acid was found to have an opposite 

 effect. 



Professor Stokes says : — " By combining a negative or concave 

 lens of ter-borate of lead with positive lenses of crown and flint, or 

 else a positive lens of titanic glass with negative lenses of crown 

 and flint, or even with a negative of very low flint and a positive of 

 crown, achromatic triple combinations, free from secondary colour, 

 may be formed without encountering (at least in the case of titanic 

 glass) formidable curvatures, and. by substituting at the same time 

 a titanic glass for crown and a borate of lead for flint, the curvature 

 may be a httle further reduced." — ' Kep. Br. Ass.,' 1862 ; Trans- 

 actions of Sect., p. 1. 



G. W. KoYSTON-PlGOTT. 



