38 PROCEEDINGS OF SECTION A. 



exti'emely tiring and difficult. Photography comes in as a great 

 aid in some researches in this direction, chiefly because the 

 sensitised plate will receive and record impressions that our ej'es 

 fail to grasp. So for telescopes and spectroscopes we ask more 

 light or larger apertures, better glass, and more perfect surfaces. 



Photography has for a long time been an indispensible hand- 

 maiden to the astronomer, and great things have been accomplished 

 by its aid, more especially in the departments of solar and stellar 

 physics ; it promises to be of immensely wider importance as 

 progress in the art is secured, not only in the departments of 

 physical but mathematical astronomy also. We are now preparing 

 for a great work, in which photography will be the chief 

 implement. I refer to the proposal to make a complete photo- 

 graphic chart of the heavens, not only of all stars visible, but of 

 all that can be seen with moderately powerful telescopes, whose 

 numbers will probably amount to several millions if all down to 

 the 15th magnitude are included, as is proposed. You already 

 know that arrangements have been made by which various 

 nations have agreed to join in this great undertaking, each one 

 taking a portion of the celestial sphere as his field of operation. 

 The preliminaries are nearly all arranged. The character and 

 size of the instrument, the kind and dimensions of the sensitive 

 plate, have been decided upon, and it is expected that most of the 

 instruments will be in their place, and work commenced some- 

 time next year (1889). It is thought we may complete this work 

 in ten years, and quite possibly in less. There is an interesting- 

 point in connection with this comparatively new astronomical 

 method arising out of the power possessed by the photographic 

 film of recording impressions far too faint to stimulate the eye to 

 vision. It has been found in photographs of stars that the latter 

 are frequently recorded on the plate that cannot be seen by the eye 

 even in powerful telescopes, because the light they emit does not 

 come within its limit of wave period, although it suffices to 

 decompose the silver film. It is highly probable, thei^efore, that 

 one of the results of this great work will be the discovery of 

 numbers of bodies, dark stars and nebulae, minute planetoids, and 

 perhaps even comets and meteroids, that have so far escaped 

 detection through the limitation of the powers of the eye to light 

 of certain wave lengths only. 



The requirements, optical, chemical, and mechanical, for the 

 successful prosecution of this undertaking would have been 

 considered a few years ago almost insurmountable. As regards 

 the telescopes and photographic films they seem to be already 

 met. The great mechanical difficulty, however, of keeping a 

 large heavy telescope following the motion of the earth so 

 precisely as to keep a star bisected by a spider web at its focus 

 for one, two, or three hours together has presented a serious 

 difficulty, which until very recently was feared would embarrass 

 the undertaking. 



