The Astronomical Outlook 



number of small telescopes from five to eight 

 inches in diameter have already been constructed 

 which are said to be very fine; and an English 

 firm now advertises its readiness to supply "photo- 

 visual" object-glasses as large as twenty inches 

 in diameter.* 



The peculiar name is given to indicate that 

 these new lenses bring the rays which are spe- 

 cially effective in photography to the same focus 

 as those which chiefly affect the eye, so that such 

 a telescope is equally useful for both photographic 

 and visual observations. 



It may be that the new century is to bring in 

 a new era in telescope-making, and that the 



* It may be permitted here, I hope, to refer to the heavy 

 loss which astronomy has /ately suffered in the dying out of 

 our great American telescope-makers, the Cambridge Clarks, 

 the father and his two sons, who, during the last twenty-five 

 years, have made more great object-glasses than all the 

 other opticians of the whole world. Among their produc- 

 tions are the largest lenses of all, now mounted at the Lick 

 and Yerkes observatories. Others, perhaps, may have 

 possessed a profounder knowledge of optical mathematics, 

 and perhaps an equal skill in the working of optical surfaces 

 to theoretical curves, but none, I think, have had so ready 

 a perception of just the right and best thing to do in order 

 to overcome or evade the difficulties caused by slight imper- 

 fections in the material, such as are sure to be encountered 

 in even the best specimens of the glass-founder's work. None 

 certainly have surpassed them in the excellence of their 

 finished lenses. 



We still have opticians, however, who are following hard 

 in their footsteps and have the advantage of the experience 

 of their predecessors; we may well hope, therefore, that our 

 country will yet be able to retain her pre-eminence in. this 

 important line of scientific art. 

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