314 Sir Howard Grubb [May 25, 



contrivance. Possibly this apparatus in its present form may be 

 capable of improvement, but it is at least a step, and a very impor- 

 tant step, in the direction of solving one of the most troublesome and 

 difficult problems to be met with in the attempt to obtain a really 

 satisfactory mounting for reflecting telescopes. 



That the reflecting telescope is capable of doing excellent work in 

 the hands of those who take sufficient cure and trouble with the 

 adjustment and in the working, is sufficiently evidenced in the results 

 obtained by Diaper, De la Rue, Common and Roberts, but the fact is 

 that to obtain good results with the reflector in its present imperfect 

 state of development, more labour and patience is required than most 

 observers care to bestow on the work, and there is much to be said in 

 excuse for this, for if an astronomer's time be taken up with the neces- 

 sary attention to the details of his instrument he will not be able to 

 pay that undivided attention otherwise possible, and at all times 

 desirable, for his more legitimate work in the obtaining of results 

 with that instrument. The fact that the reflector brings all rays of 

 light to a common focus, irrespective of their wave-lengths, while 

 the refractor is at best but a compromise, tells strongly in favour of 

 the ultimate success of the reflector over its rival. 



True, it may be said that the experiments iti glass-making that 

 have been carried on for some years at the Jena glass manufactory 

 may yet eventuate in producing qualities of glass which will remove 

 this reproach from the refractor, and enable us to perfectly balance 

 the chromatic error, and at the same time be of a sufficiently per- 

 manent character to justify its use in the case of large objectives. 

 No one would be foolish enough to attempt to make a large objective 

 of any material which was not known from previous experience to be 

 capable of preserving its perfection of surface for at least 20 or 25 

 years. 



Unfortunately there is no test of permanency except the lapse of 

 years ; even if therefore some such glass were in existence at the present 

 time, no maker who had any desire that his name should live after 

 him in his work, would care to use this untried material until actual 

 experience proved its character for permanence. This, and the fact 

 that it has not yet been found possible to produce perfect pieces of 

 this Jena glass of one-tenth of the weight of those already produced 

 of the more ordinary varieties of optical glass, cuts off any hope we 

 m glit otherwise have of being able for the present to produce large 

 objectives with perfect correction for the chromatic aberration, and 

 so long as this is the case, the reflector, which treats rays of all 

 refrangibilities alike, has in this respect the advantage. 



When we consider that the largest optical discs ever yet pro- 

 duced, and which were rightly considered a perfect triumph of art, are 

 only 40 inches in diameter, and that on the other hand Lord Kosse's 

 reflector of 72 inches diameter is now half a century old, it is tolerably 

 evident that for the present, at least, we must look to reflectors if we 

 want to increase to any large extent the power of our telescopes. 



"With this view, and understanding that it is likely an attempt 



