ASTRONOMY WITH AN OPERA-GLASS. 



74 5 



opera-glasses are made with achromatic objectives. But there are 

 great differences in the quality of the work. If a glass shows a col- 

 ored fringe around a bright object reject it. Let the diameter of 

 the object-glasses, which are the large lenses in the end farthest from 

 the eye, be not less than an inch and a half. The magnifying power 

 should be at least three diameters. A familiar way of estimating the 

 magnifying power is by looking at a brick wall through one barrel of 

 the opera-glass with one eye, while the other eye sees the wall with- 

 out the intervention of the glass. Then notice how many bricks seen 

 by the naked eye are required to equal in thickness one brick seen 

 through the glass. That number represents the magnifying power. 



The instrument used by the writer in making most of the observa- 

 tions for this sketch has object-glasses 1*6 inch in diameter, and a 

 magnifying power of about three and one half times. 



See that the fields of view given by the two barrels of the opera- 

 glass coincide, or blend perfectly together. If one appears to par- 

 tially overlap the other when looking at a distant object, the effect is 

 very annoying. This fault arises 

 from the barrels of the opera- 

 glass being placed too far apart, 

 so that their optical centers do 

 not coincide with the centers of 

 the observer's eyes. 



Occasionally, on account of 

 faulty centering of the lenses, a 

 double image is given of objects 

 looked at, as illustrated in the ac- 

 companying cut. In such a case 

 the glass is worthless ; but if the 



effect is simply the addition of a small, crescent-shaped extension on 

 one side of the field of view without any reduplication, the fault may 

 be overlooked, though it is far better to select a glass that gives a per- 

 fectly round field. Some glasses have an arrangement for adjusting 

 the distance between the barrels to suit the eyes of different persons, 

 and it would be well if all were made adjustable in the same way. 



Don't buy a cheap glass, but don't waste your money on fancy 

 mountings. What the Rev. T. W. Webb says of telescopes is equally 

 true of opera-glasses : " Inferior articles may be showily got up, and 

 the outside must go for nothing." There are a few makers, whose 

 names stamped upon the instrument, may generally be regarded as a 

 guarantee of excellence. But the best test is that of actual per- 

 formance. I have a field-glass which I found in a pawn-shop, that 

 has no maker's name upon it, but is quite capable of bearing criti- 

 cal comparison with the work of the best advertised opticians. And 

 this leads me to say that, by the exercise of good judgment, one may 

 occasionally purchase superior glasses at very reasonable prices in the 

 vol. xxx. — 48 



