The President's Address. By H. J. Slack. 115 



Mr. Dallinger has pointed out that fine dry lenses are still 

 necessary for many cases in which the use of fluids is objectionable, 

 and this brings us to the consideration of the direction in which we 

 must look for further progress. 



It is now several years since Dr. Pigott called attention to the 

 practical importance of the residual errors of the best objectives 

 then made, and he proved experimentally that with the existing 

 materials it was possible to reduce them. Since then important 

 advances have been made, both with dry and wet lenses, and it is 

 probable that for a further advance, to any important extent, the 

 optician must be supplied with fresh materials. 



Professor Abbe says, " The impossibility of removing each chro- 

 matic diiierence of spherical aberration has its root in the circum- 

 stance that, in the kinds of crown and flint glass at present provided, 

 the dispersion always goes hand in hand with the mean index of 

 refraction, in such a way that the higher dispersion is attached to 

 the highest index. The outstanding aberrations might be com- 

 pletely, or very nearly, compensated, if an optical material were 

 provided in which a relatively lower refractive index were united 

 with a higher dispersion, or a higher refractive index with a rela- 

 tively smaller dispersion. It would then be possible by combining 

 such materials with the ordinary crown and flint glass to remove 

 the chromatic and spherical aberrations which are partly discon- 

 nected, and thus fulfil the essential conditions arising from the 

 chromatic difierence." * 



Professor Abbe also remarks upon the very small number of 

 persons engaged in the manufacture of glass for optical purposes, 

 and upon the few substances that have hitherto been employed in 

 its preparation. Looking to the present conditions of manufacturing 

 industry, which ofier the largest gains to those who can produce at 

 the lowest price articles required in great quantities, it is not pro- 

 bable that purely commercial considerations will induce anyone to 

 devote attention to the demands of science for new kinds of glass 

 or substitutes for glass, and the task is not likely to be undertaken 

 unless through the help of private munificence, or State aid. 



Besides glasses, which are chiefly silicates of potash, soda, and 

 lead, with a little alumina or lime, &c., there seems a probability 

 that a class of compounds resembling precious stones may come into 

 optical use. IM. Feil, a celebrated manufacturer of optical glass, 

 and M. Fremy have succeeded in artificially producing rubies and 

 sapphires, alumina minerals of high refractive power, and it is not 

 impossible some quite new compounds may be formed. When Sir 

 David Brewster experimented with jewel lenses, he spoke very highly 

 of garnet, which is a compound of three or four silicates, chiefly of 



* ' Die Optischeu Hiilt'sunttel der Mikroskopie,' von Dr. E. Abbe, Professor 

 un der Universitat zu Jena, liraunschweig, 1878. 



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