ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 299 



thirty years. Not only are the present kinds of crown and flint glass 

 produced in formerly nnattained perfection, as regards purity, homo- 

 geneity, and freedom from colour, but the whole series of optical glass 

 has been widely extended in one direction by the manufacture of flint 

 glass which considerably surpasses the previous kinds in high re- 

 fractive power and dispersion. This progress, however, is all in the 

 direction of inherited tradition. The art of glass-making has not 

 apparently started on a fresh path, to enrich practical optics with 

 new materials, and from the lack of earnest competition, the business 

 interests of the proprietors of this manufacture do not offer any special 

 incentive to the pursuit of ends which do not promise them assured 

 advantages. Further, let us reflect how dangerous it is, that a branch 

 of industry so important and so indispensable to many sciences should 

 be in the hands of the few, so to speak, for under these circumstances 

 unfortunate coincidences might threaten its continuance, and occasion 

 a serious calamity. It is therefore a vital question for optical and 

 other sciences interested therein, that in the future more forces should 

 be gathered into the field, and that a keener competition should call 

 forth stronger incentives to progress. 



We can scarcely suppose that private initiative will suffice to 

 supply this need without a strong external impulse. Undertakings 

 of this kind are attended with so much difficulty and necessitate so 

 large an outlay for results, which even under favourable circumstances, 

 are so remote, that they can have little attraction even for enter- 

 prising people. A great rise in the industry in question can scarcely 

 be expected unless funds are freely granted for its furtherance by 

 Corporations or the State. The field is open here for learned 

 societies which are in a position to offer material help towards the 

 needs of science, to perform a most beneficial and worthy task. For 

 great and various interests are dependent on the increasing efficacious- 

 ness and progress of the glass-manufacture. It is not, by any means, 

 the Microscope alone which is here considered, but all arts and 

 sciences dependent on the use of optical resources. 



A retrospect of the last portion of this discussion on the ways 

 and means of perfecting the Microscope in the future, shows a more 

 favourable prospect than the earlier considerations. As regards that 

 part of the performance of the Microscope which touches the 

 dioptrical functions of the objectives, an increasing improvement of 

 the instrument in important points may be expected in the future. 

 The difficulties which at present oppose further progress in this 

 respect, and will perhaps long continue to do so, need not in any way 

 be considered as insurmountable. This is the proper field in which 

 optical art may hope to attain further results. The question of the 

 best adapted and most advantageous means of solving the difficulty 

 under consideration is certainly not exhausted either as regards theo- 

 retical optics, or those practical arts which co-operate in the work 

 of opticians. Theory may, in time, by a deeper insight into optical 

 problems, point out new methods of removing, more effectually than at 

 present, the chromatic dispersion and spherical aberration in objectives ; 

 practical optics may, by the perfecting and refining of the method of 



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