﻿80 Intelligence and Miscellaneous Articles. 



the opening be passed over the eye-tube and suffered to close by its 

 elasticity, the desired result will be secured. 



From my own experience I feel confident that, for certain special 

 orders of research, the use of eye-screens of this sort (the details 

 being arranged according to convenience) will be found of great 

 service. I may particularize observation on moonlit nights, the 

 examination of faint nebulae or the search for them, the search for 

 minute points of light, delicate spectroscopic and polariscopic re- 

 searches, and the investigation of questions concerning the colours 

 of the planets, of the lunar regions, &c. The following description 

 of the modus operandi will serve to supplement the above general 

 sketch. The equatorial being set on any delicate object, the ob- 

 server fastens the screen over his eyes, attaches the sleeve-curtain 

 over the eye he proposes to work with, slides the elastic ring over 

 the eye-tube, and suffers it to close round that tube. Having seen 

 that the object is in the field and the rate of driving just, he closes 

 both the eye-doors, keeping his eyes open and directed on the per- 

 fect blackness of the velvet lining. When he thinks the eye suffici- 

 ently prepared, he draws out the sliding eye-door over the telescope- 

 eye, and is then able to apply the full powers of his eyesight (under 

 exceptionally favourable conditions) to the examination of the object 

 he has to deal with, 



I am desirous of learning how far the efficiency of such contri- 

 vances depends on aperture — that is, whether the performance of 

 large telescopes is as much improved as that of smaller instruments. 

 If any Fellows of the Society should care to experiment on this 

 matter (which is not unimportant, I conceive), they would be confer- 

 ring a favour on me by communicating the results they may obtain. 

 I refer here simply to the comparison between the work of a telescope 

 used in the ordinary way and when supplemented by eye-screens. 



I may renew here my suggestion of the great importance of ap- 

 plying such contrivances during the examination of the corona in 

 total solar eclipses, whether with the telescope, spectroscope, or 

 polariscope. As regards the spectroscopic analysis of the corona, 

 any method by which the visibility of a very faint continuous spec- 

 trum can be increased (and I imagine that eye-screens such as I have 

 proposed would have such an effect) cannot but be well worth 

 trying. It seems highly probable that the variations in the accounts 

 given so far depend wholly on the amount of opening in the slit — a 

 narrow slit giving a very faint continuous spectrum and three some- 

 what brighter lines (one can scarcely speak of bright lines in this 

 case), which are under these circumstances very difficult and delicate 

 objects of observation ; a somewhat wider opening gives a continu- 

 ous spectrum, bright enough to obliterate two of the lines, but leav- 

 ing one (the one Professor Harkness saw) fairly visible ; a wider 

 opening shows only a continuous spectrum, the lines being all ob- 

 literated. This at least is the explanation which seems alone con- 

 sistent with the principles of spectroscopic analysis ; and it is ob- 

 vious that, if it be just, any increase in the eye's power of apprecia- 

 ting faint light must be a great gain. — Monthly Notices of the Royal 

 Astronomical Society, November 11, 1870. 



