Dkcembeb 1, 1900.] 



KNOWLEDGE. 



275 



to fonii part of a roniplicat-ed but well-dcfincd whorl 

 of stars. 



It seems quite clear that though the general form 

 and stnieturo of the Milky Way in Cygnus is well shown 

 on this plate, no true nebulosity is shown by it over its 

 breadth, but only apparent nebulosity due to the 

 aggregation of small discrete stars too faint to be 

 separately perceived by the unaided eye. It is possible 

 that true nebulosity is photographed in the region 

 round 60 Cygni, but that cannot be decided from this 

 photogi-aph alone. Possibly an exposure of 13 hours 

 might settle the question, and some of these years I 

 shall hope to tiy it. — if tlie Clerk of the Weather will 

 some August screw the hand of his barometer to " Set 

 Fair," and go on his summer holiday, and the moon is 

 new ; and my neighbours will go to bed early, and turn 

 out the gas. 



♦ 



Hftrrrs. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



THE 'COLLINS MOX'OPLANH TELESCOPH." 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — The " Collins' Monoplane Telescope," described 

 on page 252, appears to be a combination of a poor 

 Newtonian reflector with a poor refractor. The only 

 advantage? claimed over the ordinai-y refractor arc 

 perfect achromatism and shortness, both which the 

 Newtonian already possesses, without the gi'oat disadvan- 

 tages of the Collins. The Newtonian requires two pieces 

 of glass of ordinary quality, and has only two surfaces 

 to be worked, one to a perfect cui-ve, the other to a 

 perf2ct plene, both fairly easy to test for figure, and the 

 reflection is from surfaces of silver not covered with 

 glass, and therefore brighter than any glass-covered 

 surface, and there is no absoi-ption of light by pa.ssing 

 through glass. 



The Collins requires two oj)tically perfect discs of 

 glass, of which the second must be so homogeneous that 

 I fear practically they would be difficult to produce 

 and exceedingly costly. If used with a silvered flat it 

 has four glass surfaces at which light is lost, two lenses 

 absorbing light in their substance, one of which it 

 traverses twice, one silvered surface less reflective than 

 in the Newtonian, and another under similar conditions 

 as in the Newtonian. If a prism is used instead of a 

 flat, there are six glass siu'faces each taking a percentage 

 of the light away, and the equivalent of five absorptioa-s 

 in the thickness of the glass. The glass surfaces have 

 all to be worked perfect, and at least in the case of 

 the correcting lens-mirror local correction appears to 

 me impracticable. With ordinary refractors on a large 

 scale, local correction is, I believe, a vei-y important 

 matter, and it is only on a large scale that the Collins' 

 Monoplane's advantage of shortness would be of value. 



I doubt if a good telescope of any large size could be 

 constructed on the lines given ; I doubt if it would be 

 perfectly achromatic ; and I am sure it could not equal, 

 much less surpass, a Newtonian of the same aperture. 

 Why then go to so much expense and trouble ? 



Edwin Holmes. 



[Mr. Holmes's comdemnation of the " Monoplane " 

 principle is far too off-hand. The idea is being made 

 the subject of the most careful experiment and investi- 

 gation, both in this country and on the Continent; and 

 so far from dismissing it in Mr. Holmes's summai-y 

 stvle, one of the highest authorities in practical optics 

 in Europe is now having a large telescope constnicted 

 upon these lines. — E. W.alter Mau.nder,] 



I'lli: I'll \SE (1F VENUS SEEN WITH THE 

 N.VKEO EYE. 



TO THE EDITORS OF KNOWLEDGE, 



Sirs, — To see our nei;j;hbour Venus in brilliant sunshine 

 when near her greatest elongations is, I am quite aware, 

 nothing remarkable as a feal, but my recent experience in 

 this connection is to myself quite unique, and it.s relation 

 may prove not uninteresting to your readers. Living as 

 I have for years jiast in north-west Loudon, I have perforce 

 had to observe sun, moon, and planets, as through the 

 u]iper rea;hes of the dusty air of the mctrojiolis. I have 

 often noti<'ed how much more transparent my air is in the 

 month of October than it is generally at any other time of 

 the year. Several tinii-s during the last fortnight the 

 morning air has struck me as exceptionally clear, so much 

 so that I remembered Venus, whose movements I had 

 ceased to folk)W since her last inferior conjunction, and I 

 determined to look her up in bright sunshine. No sooner 

 resilved upon than accomplished. The first occasion was 

 on the 5th instant, between 7 and 8 a.m., the sun low but 

 brilliant. On the morning of the 14th instant I saw her 

 again in bright sunshine at Oh. lum., or soon aft<'r her 

 meridian passage. At these times, as at some intermediate 

 ones, I was chiefly interested in noting her lustre. SIi<! 

 did not appear as a white point on a ilark l)lue sky, Imt 

 as sparkling against a rather pale blue background. Now 

 this morning (October Liith) I observed her several times, 

 the last occasion being as late as !)h. .50m. As before, 

 sunshine was as plentiful as one could desire. On no 

 adventure of this kind have I employed any aid to find 

 her— optical or mechanical. I have simply stood in the 

 shade of the meteorological instruun'nt scre<'u. Very well, 

 then, to-day she has a]ii)eared as a small white disc — 

 spurious, of course. Until about an hour ago I did not 

 know thai Venus had passed her greatest western elongation 

 — in fact I did not think she had ; hence I adjudged her a 

 phase about equal to that of a sis days old moon. But I 

 see by the Nautical Almanack this evening that on this 

 identical day 0-(537 of her disc is illuminated, and this is 

 where the curiosity enters. Her angular diameter is about 

 18 4", and her distance from the earth this morning was 

 something like 84,8941)00 miles. It is laid down as a 

 physiological fact that the smallest object to which 

 unassisted human vision can assign a definite shape must 

 subtend an angle of about one minute of arc. A bright 

 point is an entirely different matter. Now I know my 

 own sight is by no means marvellously sharp, so that I 

 am greatly puzzled to know how the illusion of the disc 

 originated ; because, as to illumination — we see nothing at 

 all if there is no light to see it by. It may be that the 

 s|)urious disc of a star in a telescope offers an exjilanation ; 

 in fact, I was satisfied to accept that as such before I 

 found this evening that nearly two-thirds of the planet's 

 disc was illuminated this morning. But now the question 

 arises : is it in any circumstances possible that a body 

 subtending so small an angle could be seen to possess a 

 definite shape, or how in such a case is the illusion — if 

 such it l>e — caused ? I may add my eyes seem purely 

 emmetropic, and whilst on the one hand there is no 

 suspicion of myojiia, on the other there is nothing 

 miraculous about tiiem. William Godden. 



38, Burrard Road, 



West Hampstead, N.W., 

 1.5th October, 1900. 



'DARK MARKINGS IN THE SOLAR COEONA." 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — lu reading Mr. Wesley's admirable article on 

 the " Dark Markings in the Solar Corona " (Kxowledoe, 



