6 Introduction. 



microscope lenses to telescopic objectives. The scientific investigations on the 1 

 correlation of the optical properties of glass and its chemical constitution, which 

 formed the starting point of the smelting of glass in Jena, have at the same 

 time resulted in a considerable extension of the number of glasses which may 

 be applied for purposes of practical optics, and opticians have now a series of 

 new types at their command which vary considerably from the older crowns and 

 flints with respect to refractive and dispersive power. This extension of the 

 range of available material led in several provinces of practical optics to valuable 

 improvements, which naturally originated in Germany. Telescopical optics is 

 that department which hitherto has derived the least benefit from the new 

 glasses. Partly on account of the difficulties attending, with some of the new 

 glasses, the production of large discs sufficiently free from defects and partly 

 because of the aptitude of these fusions to yield to atmospheric influences it 

 has been deemed wiser to continue to employ the older types of crown and 

 flint glass for the manufacture of telescopic objectives. - - In the construction 

 of photographic lenses and of the microscope the greatly extended range in. 

 the refractive and dispersive powers of the glasses has, in the mean time, given 

 rise to many practical successes in valuable conbinations, which with the older 

 material could not possibly be produced. In the case of photographic lenses- 

 this success is mainly due to to the fact that with the new glasses achromatic 

 doublets may be constructed in which at will the positive or the negative member 

 may be made to have the higher refractive index, whilst with the older material 

 achromatization of a collective lens always assigned tt& higher refractive index 

 to the negative, in the case of a dispersive lens to the positive member. The 

 removal of this limitation, which was mainly effected by the introduction of the 

 Jena baryta-glasses, has resulted in a series of new photopraphic combinations. 

 In the case of the microscope the conditions for the utilization of the progress 

 made in glass-smelting were much more favourable; for in this case also such 

 fusions could be employed which are obtainable only in relatively small quantities r 

 and furthermore the material is in a much less degree required to resist external 

 influences. Consequently, even such glasses as the phosphate and baryta-glasses- 

 have been largely employed in microscopic lenses though, for those two con- 

 siderations, entirely out of place in other combinations. These new glass-types 

 have furnished the means for vast improvements in the chromatic and spherical 

 correction of microscope lenses and in the increase of the magnifying power of 

 the ocular (Apochromatic objectives and compensating oculars). Very important 

 is also the German manufacture of spectacles, opera-glasses, stereoscopes 

 and similar instruments. The extensiveness of this branch of industry is 

 perspicuously illustrated by the fact that in one single town, Rathenow, there 

 are 94 independent optical manufacturing concerns employing 1035 persons. - 

 It is not surprising that Germany, being the home of spectrum-analysis, holds 

 a prominent position in the manufacture of spectroscopic apparatus. A number 

 of large optical and mechanical firms are engaged in the manufacture of spec- 

 troscopes and many of these are exported 'to all parts of the world. They range 

 from the largest and finest instruments for astronomical and physical research 

 for chemical and photographic investigations down to the smallest hand spec- 

 troscopes. German Polarization appliances also are famous. We may here 

 mention, beside the apparatus used in chemical and physical examination, in 



