THE COMING TOTAL ECLIPSE OE THE SUN 11 



the most important feature is the focal length or the size of the 

 telescope. Since the photographic image of the corona will not 

 bear magnifying without dispersing the available light, and thus 

 blurring out the details of the picture, which is the most important 

 feature to retain to the utmost, one can not use a short telescope 

 and at the same time a magnifying eyepiece to enlarge the image 

 by projection on a screen or on a photographic plate. The only 

 alternative in order to get an image of large diameter is to use a 

 long-focus lens. The effect of a difference of focus upon the image 

 of the corona is well shown on Chart V, which gives a small corona 

 (1) taken with a four-foot lens (Barnard), (2) with a fifteen-foot 

 lens (Pickering), and (3) with a forty-foot lens (Schaeberle). The 

 diameter is proportional to the focal length, but the difference of 

 effect upon the details is very important. In the small picture the 

 details of the corona near the sun are completely lost in the gen- 

 eral light, while the coronal extensions from the middle latitudes 

 are seen at a great distance from the sun — as much as one million 

 miles; at the same time the polar rifts are distinctly marked, so 

 that the pole or central line from which they bend is readily lo- 

 cated. On the second picture the details of the polar rays are 

 better brought out, but the extensions are shortened. In the third 

 the region near the sun's edge has many interesting details very 

 clearly defined, while all the extensions are gone. It is evident 

 that each lens has its advantage, according to the details sought, 

 and they ought all to be employed in the eclipse. The reproduc- 

 tions on paper by no means do justice to the original negatives, 

 which make the distinctions even more pronounced than shown on 

 Chart Y. 



Some amateur observers have telescopes but no mountings suit- 

 able for eclipse work, and many astronomical telescopes have good 

 equatorial mountings at home which are yet unavailable in the field 

 ,for lack of proper foundations and supports. The ordinary tele- 

 scope balanced near the center, with the eye end subject to all sorts 

 of motions which may happen through jarring and rough handling 

 in the hurry of shifting photographic plates, makes a very poor 

 eclipse apparatus. All telescopes of any length should be held 

 firmly at each end, so as to be perfectly steady, since the least 

 vibration ruins a coronal picture devoted to delicate photographic 

 effects. There are two ways of accomplishing this, and only two. 

 Dispensing with an equatorial mounting, the lens must be set per- 

 manently on a base, and light reflected from a mirror must be 

 utilized, which shall be concentrated on a plate also placed on a 

 fixed base. This is the method employed by Schaeberle in Chili, 

 April 16, 1893, to obtain No. Ill of Chart V. There is no objec- 



