104 THE HISTORY OF THE TELESCOPE. 



guide us in valuing the results of the subsequent efforts of Lassel and 

 Eosse. The same line of study would bring us to Grubb's clever and 

 interesting equatorial mounting of that anachronism, the 4-foot Mel- 

 bourne reflector. But we should find nothing of very notable interest 

 in the mounting of refractors, after the time of Huyghens and Hook, 

 until Fraunhofer invented ii type of mounting for the famous Dorpat 

 equatorial, which still remains in its essential features as the type in 

 universal use. With the increase in size of the telescopes to be directed 

 towards the heavens, however, the number and complexity of the me- 

 chanical problems to be solved has been vastly increased, so that they 

 have taxed the best powers of some of the ablest mechanicians. The 

 Eepsolds, of Germany, and Sir Howard Grubb, of Dublin, have specially 

 distinguished themselves in this field of activity. But it seems to me 

 that none have shown greater fertility of resources, greater skill in the 

 solution of every problem affecting the comfort and efficacy of the ob- 

 server, and greater taste, combined with accurate workmanship, than 

 have the celebrated firm which has mounted the telescope at Mount 

 Hamilton and that at Carleton College, 



We come now to a consideration of the present state of the art of 

 lens-making. We ask why such a very large proportion of the tele- 

 scopes in existence are bad; why there was a time, brief it is true, dur- 

 ing which the glass-maker was certainly in advance of the demands of 

 telesco])e-makers; and why, finally, the first of the great modern objec- 

 tives was in the hands of the most skillful optician in Great Britain for 

 seven years, and even then this maker asserted that it Avas incom])lete. 



These questions can not be answered in a word, but we can, at least, 

 gain much in persjjicuity by recognizing that the reasons are of two 

 distinct kinds, namely, purely technical, and theoretical; and by re- 

 garding them briefly in succession. 



The art of lens-making can be certainly traced back to the 13th cen- 

 tury, though the methods at a much later day than that were so rude 

 that, as we have seen, Galileo had the utmost difliculty in making a 

 lens good enough to bear a magnifying power of 30 times. At the 

 ])resent day there is little difficulty in selecting a spectacle glass which 

 would rival that most famous of all telescopes. Not until after another 

 generation of effort was there such notable improvement in the tech- 

 niijue of lens-making that farther astronomical discovery was i)ossil)le. 

 The reasons for this slow progress are to be found in the extremely 

 critical requirenuMits for a good lens. A departure by a fraction of a 

 hnndrcd thousandth part of an inch from a correct geometrical surface 

 will greatly impair the performance of an objective. But even at this 

 day the limit of accurate measurement may be set at about a one-hun- 

 dred-thousandth of an inch, while it is (juite probable that ten times 

 that value was vanisliingly small to the artisans of a century or more 

 ago. It was necessary therefore to devise a method of polishing — 

 for it is a comparatively simple matter to grind a surface accurately — 



