298 SUMMABY OF CURRENT RESEARCHES RELATING TO 



higher dispersive power, or a higher refractive index with a relatively 

 lower dispersive power. It would then be possible by proper com- 

 bination of such materials with the usual crown and flint glass, to 

 partly remove the chromatic and spherical aberrations, independently 

 of each other, and thus fulfil the essential conditions on which the 

 removal of the chromatic difference depends. 



As the defects of the present objectives, in regard to the chromatic 

 as well as the spherical aberration, originate in the optical properties 

 of the substances on which the optical art of the day is based, 

 the further perfecting of the Microscope in its dioptrical working, is 

 therefore chiefly dependent on the progress of the art of glass-making, 

 and will in particular require, that new hinds of glass should he pro- 

 duced, which admit of a better correction of the so-called secondary 

 spectrum and which show a different relation of the refractive to the dis- 

 persive power than at present has been obtained. 



The hope that such claims can be satisfied, in the more or less 

 distant future, and the way opened for a substantial perfecting of 

 the Microscope, as well as of the other optical instruments, rests on 

 thoroughly established facts. The mode in which, in the kinds of 

 glass now used, the indications of refraction and of chromatic dis- 

 persion appear, need not be considered as a natural necessity. For 

 a sufficient number of different transparent substances may be chosen 

 from amongst natural minerals and out of the many artificially 

 formed chemical compounds, which offer essentially different properties 

 as regards their refraction and chromatic dispersion, only that in 

 other respects they are not adapted for optical use. Experiments for 

 the manufacture of glass with less secondary dispersion, which were 

 undertaken several years ago in England, with the co-operation 

 of Prof. Stokes, although they were without practical result, gave 

 noteworthy suggestions on the specific effect of certain bases and acids 

 on the refractive properties. The uniformity which the present kinds 

 of glass show in their optical properties, is to be attributed to the fact 

 that the glass factories have hitherto used only a small number of 

 materials, scarcely any other than aluminium and thallium, besides 

 silica, alkali, lime, and lead, and we might reckon with some confidence 

 on a greater variety of production, if only the glass manufacturers, led 

 by methodical study of the optical properties of various chemical 

 elements in their combinations, would leave that very limited field. 



Unfortunately there seems little hope under present circumstances 

 of any important advance in this direction in the immediate future. 

 The present prospect, on the contrary, indicates a state of affairs which 

 endangers many scientific interests. The manufacture of optical 

 glass has been for a long time not far removed from a kind of 

 monopoly ; at least the art is in the hands of so few, that competition 

 is out of the question. Since Daguet's glass-works were closed, there 

 are now only two such institutions, which supply the general 

 demand, while the third, founded by Utzschneider and Fraunhofer — 

 the only one in Germany — has remained exclusively in the service of 

 one optical workshop. It must, it is true, be admitted that this art 

 has made very important progress in many respects during the last 



