88 S. P. Langley—Minute structure of the Solar Photosphere. 
exception of some early ones by Lockyer and Huggins (a paper 
by the latter being accompanied by a suggestive drawing),” an 
of some illustrated by Secchi’s excellent sketches. The diffi- 
culties of observation are in fact extreme, they are enhanced by 
want of a precise nomenclature, and their elucidation is not 
easy without elaborate drawings. The present article, which 
partly anticipates a prospective and completer publication on 
the part of the Allegheny Observatory, is meant as a help to 
their removal, and as a contribution of material for a judg- 
ment upon the theories of the solar circulation due to Faye, 
Kirchoff, Lockyer, Secchi, Young, Zéllner, and other eminent 
students of Solar Physics. As it is based only upon direct 
telescopic study of the minute components of the photosphere, 
it will offer a distinct point of view from that taken by the 
spectroscopist, and though therefore incomplete, its testimony to 
the action of solar currents will yet have a certain value from 
the fact of this independence. 
Most of the minuter phenomena to be described are un- 
recognizable except with large apertures, which, in spite of 
the abundant light, are indispensable for reducing irradiation. 
That of the equatorial of the Allegheny Observatory is thir- 
teen inches, but when we use so large an instrument on the 
sun, we find two principal difficulties. The first, arising from 
the concentrated light, can be mitigated by optical aids. By 
the liberality of a citizen of Pittsburgh, the equatorial has been 
supplied with several special accessories for such study, the 
most essential of which is the polarizing eye-piece, which, in the 
form devised and employed here, presents a solar image free 
from color and of any brightness desired. To secure the rapid- 
ity needed in sketching objects whose forms are so changeable 
without sacrificing accuracy, Mr. Rogers, of the Harvard College 
Observatory, has had the goodness to rule for me one of his very 
exact reticules on thin glass, which for the present purpose 18 
divided by faint lines into squares, the side of each being one 
one-hundredth of an inch, the value of which in the solar focus 
is 11°55. The Filar micrometer is of course also employ 
where its use is appropriate. 
The second and more formidable difficulty arises from the 
disturbance of our own atmosphere, and for this there is 00 
present remedy but assiduity and patience. | 
It may be stated generally that it will be useless to look for 
the minuter details hawstied whatever the power of the tele- 
scope, save in exceptionally good definition. In such an inves: 
tion, drawings are at present indispensable. Photo raphy 
in the hands of Messrs. De la Rue and others has been of grea 
service in furnishing unquestionable data for the student of the 
* See Monthly Notices Roy. Ast. Soc.; also, Proctor, “The Sun.” 
