Silvered Mirror Telescopes. 177 



errors, from Mr. With's extraordinary skill in giving them 

 the trne form. Beyond this, it may be stated that the cell 

 mounting devised by Mr. Browning, and his method of sup- 

 porting the plane mirror, or prism, secures permanent good 

 performance in any position, and closely approximates the 

 sort of definition of large stars to the neatness obtained by 

 first class and moderate-sized refractors. 



It is the disadvantages of the system that we are most 

 often requested to elucidate, and we have waited for some time 

 in order to estimate them as fairly as possible. First comes 

 the question, Is it not troublesome to keep a silvered mirror 

 telescope in order ? and we answer, Certainly not, as they are 

 made by Mr. Browning. 



A badly-mounted refractor is a great nuisance, but it is 

 nothing in abomination and vexation to a badly-mounted 

 reflector, which will keep its owner in an optical purgatory of 

 a most unpleasing kind. By making the mirrors of very thick 

 glass, and mounting them in the cells figured in a former 

 number, Mr. Browning's reflectors are much like achromatics, 

 and indeed from the glass not being thinned off at the edges, 

 as in the double convex lens of achromatics, the chance of 

 flexure is much less. Mr. Browning's cell answers perfectly 

 for sizes from 6| to 101, and would probably do well for bigger 

 instruments, though monsters might require other special con- 

 trivances to guard against flexure. There is no difficulty in 

 adjusting these telescopes if slightly deranged. The screws 

 are very manageable, and the test of true adjustment very 

 easy. The instrument would, of course not, be out of 

 order when it comes from the maker, but if the mirror is 

 taken out of the cell, and the prism or flat dismounted, 

 a very few minutes will suffice to put both right again. 

 The plan is to remove the glasses from a deep eye-piece, 

 and look through the small hole left in the brass work 

 at the prism, or plane mirror, with a Barlow lens inter- 

 posed. The eye readily detects want of centering, and the 

 motions to obtain it are simple and easily understood by in- 

 spection of the parts. Thus we do not see that any one 

 accustomed to philosophical instruments need be at all afraid 

 of the new reflectors on this ground. 



Then comes the question of keeping the thin film of silver 

 in order. On this matter we have made divers experiments, and 

 incline to the simplest treatment — that of using the mirror as 

 if it were a fine achromatic object-glass — covering it up when 

 out of use, and leaving it alone, with a rare and occasional 

 wiping. This remark applies only to telescopes sheltered in 

 some moderately good observatory. If used out of doors, it 

 would probably be advisable to take the mirror indoors in its 

 vol. xi. — no. in. n 



