PLEASURES OF THE TELESCOPE. 223 



of the rings when out of focus on either side, an experienced eye 

 can readily detect any fault that a telescope may have. The 

 amateur, of course, can only learn to do this by considerable 

 practice. Any glaring and serious fault, however, will easily 

 make itself manifest. Suppose, for example, we observe that the 

 image of a star instead of being perfectly round is oblong, and 

 that a similar defect appears in the form of the rings when the 

 eyepiece is put out of focus. We know at once that something is 

 wrong ; but the trouble may lie either in the object glass, in the 

 eyepiece, in the eye of the observer himself, or in the adjustment 

 of the lenses in the tube. A careful examination of the image 

 and the out-of-focus circles will enable us to determine with 

 which of these sources of error we have to deal. If the star 

 image when in focus has a sort of wing on one side, and if the 

 rings out of focus expand eccentrically, appearing wider and 

 larger on one side than on the other, being at the same time 

 brightest on the least expanded side, then the object glass is prob- 

 ably not at right angles to the axis of the tube and requires read- 

 justment. That part of the object glass on the side where the 

 rings appear most expanded and faintest needs to be pushed 

 slightly inward. This can be effected by means of counterscrews 

 placed for that purpose in or around the cell. But if, after we 

 have got the object glass properly squared to the axis of the tube 

 or the line of sight, the image and the ring system in and out of 

 focus still appear oblong, the fault of astigmatism must exist 

 either in the objective, the eyepiece, or the eye. The chances are 

 very great that it is the eye itself that is at fault. We may be 

 certain of this if we find, on turning the head so as to look into 

 the telescope with the eye in different positions, that the oblong 

 image turns with the head of the observer, keeping its major axis 

 continually in the same relative position with respect to the eye. 

 The remedy then is to consult an oculist and get a pair of cylin- 

 drical eyeglasses. If the oblong image does not turn round with 

 the eye, but does turn when the eyepiece is twisted round, then 

 the astigmatism is in the latter. If, finally, it does not follow 

 either the eye or the eyepiece, it is the objective that is at fault. 



But instead of being oblong, the image and the rings may be 

 misshapen in some other way. If they are three-cornered, it is 

 probable that the object glass is subjected to undue pressure in 

 its cell. This, if the telescope has been brought out on a cool 

 night from a warm room, may arise from the unequal contraction 

 of the metal work and the glass as they cool off. In fact, no good 

 star image can be got while a telescope is assuming the temj^era- 

 ture of the surrounding atmosphere. Even the air inclosed in 

 the tube is capable of making much trouble until its temperature 

 has sunk to the level of that outside. Half an hour at least is re- 



