Silvered Mirror Telescopes. 179 



rents in the tube. These may be greatly diminished by two 

 means — first, having the tube larger than the mirror; and 

 secondly, using the telescope out of doors, or in a form of 

 observatory, which permits a ready equalization of its internal 

 temperature with that of the outer air. Small revolving domes, 

 with narrow slits, maximize the difficulty, and in awkward 

 states of the weather render satisfactory performance impos- 

 sible. All sources of optical disturbance increase rapidly with 

 the size of the instrument, whatever be its construction, and 

 the larger sizes require the best conditions and the most care. 

 Under the same circumstances of disadvantage, reflectors may 

 be expected to work rather worse than refractors with close 

 tubes, though the difference is slight. 



With moderate sizes, in good weather, the chimney cur- 

 rents of the new reflectors are scarcely noticeable, but when 

 bitter east winds blow through a warmer atmosphere, they 

 become very troublesome, and on the same nights no refractors 

 will perform well. 



We cannot look upon silvered mirror telescopes as only 

 substitutes for refractors. If they have certain peculiar dis- 

 advantages, they have also peculiar merits, and those who get 

 used to them feel no desire to change. The inconveniences of 

 refractors eight, ten, twelve, fourteen feet long, are very 

 serious, while reflectors of equal power are manageable and 

 handy. A revolving eye-piece is indispensable to comfortable 

 working. With it there are no awkward positions for the 

 observer; without it many that are painful and perplexing. 

 It is a very difficult task to have all the adjustments so perfect 

 that a rotation of the eye-piece effects no displacement of the 

 image. The adjustments may be so good that stars of less 

 than 1" apart, may be readily divisible in all positions of the 

 instrument, and yet the rotation of the eye-piece may produce 

 changes in apparent declination and ascension. With given 

 adjustments these errors will be constant, and do not practically 

 interfere with equatorial finding . 



We find a Barlow lens and lower eye-pieces, as a rule, 

 better than deeper eye-pieces without it, and upon planets and 

 delicate moon objects we prefer Mr. Browning's achromatic 

 eye-pieces to the Huyghenian. A monster aplanatic of Home 

 and Thomthwaite works well with the Barlow on nebulae or 

 clusters, and on many large objects usefully without it. 



Duly balancing advantages and disadvantages, we feel 

 satisfied that the silvered mirror telescopes will prove an 

 immense boon to astronomical observers. They bring within 

 the reach of amateurs with moderate means, an amount of 

 optical power and a perfection of optical work hitherto confined 

 to a few first-class observatories; and if fine prisms are used 



