12 POPULAR SCIENCE MONTHLY. 



tion to it if an observer possesses a perfectly plane mirror, which it 

 is very difficult and also expensive to obtain; if the reflecting mir- 

 ror should be imperfect it would distort the image of the corona. 

 The second method, lacking a good mirror, is to mount the long- 

 focus lens in a tube and point it directly at the sun. A forty-foot 

 lens was thus mounted at Cape Ledo for December 22, 1889, and 

 its action was very satisfactory. Of course, it was a cumbersome 

 arrangement, and could not be employed by a small party. The 

 foundation for the mounting of the forty-foot tube consisted of 

 two casks filled with stones and cement, and set firmly in the 

 ground. These made two good piers, since the narrowing tops of 

 the casks held the bed plates of the telescope as in a vise. A tri- 

 angle, whose base was parallel to the earth's axis and having the 

 telescope tube itself for the long side, was fitted with an extension 

 rod for adjustment in altitude on the third side, and the whole 

 was made to revolve on ball bearings. This triangular support 

 was rotated by a side rod of adjustable length, whose end termi- 

 nated in a sand piston working with a regulating valve. The sand 

 flowed out steadily like an hour-glass, and dropped the tube, keep- 

 ing it central on the sun. The image was made to follow accu- 

 rately for twenty minutes without tremor, all the time holding the 

 solar disk tangent to fixed lines. The principle of a revolving tri- 

 angle and a short piston, taking the place of an expensive reflect- 

 ing mirror with a delicate clockwork or one carrying a telescope 

 balanced on its center but subject to jars and side motions, is an 

 important assistance in field work on account of its ready adapta- 

 bility to all sorts of observations. Since time is limited, it is neces- 

 sary to provide all operations with automatic arrangements as far 

 as possible, by using such an apparatus as that described. What 

 can be applied successfully to a forty-foot lens can serve for shorter 

 telescopes. In combination with spectroscopes, polariscopes, and 

 special instruments for photographing, an immense amount of work 

 can be compressed into the few seconds allowed by mounting them 

 all on such a movable frame as the triangle. The old-fashioned 

 method of putting one observer to one telescope ought to be aban- 

 doned. Of course, for a rising sun during the forenoon a modifi- 

 cation in the moving support must be employed. This should be 

 such as to cause the objective of the telescope to rise from the 

 ground toward the meridian, and it must be accomplished by at- 

 taching a heavy weight which in sinking draws the tube upward. 



Of the two kinds of photographic plates, the -wet and the dry, 

 the dry plates are much more convenient in the field, and are good 

 for certain purposes. The objections to them are that the rough 

 granulations of the gelatin film sometimes overpower the fine de- 



