May 19, 1882.] 



KNOWLEDGE 



605 



liy the vitreous medium ; nnil fjlass, 1 believe, is knuwn to absorb 

 the ultrn-red rays. 



6. A thin dinner-table water car .Se, full of pure water, transmits 

 the fiicussed sunbeam as white light. 



7. {Easter Sunday). — This fine sunny morning I really believe I 

 kave proTed my point by eliminating', or nearly eliminating, the red 

 rays from the solar spectrum thrown on a sheet of white paper, by 

 carefully introducing " the green beam " into the prism ; when, 

 instead of the broad red band, an oval spectrum appeared, full of 

 pale blui.sh light, with a broadish yellow, and an extremely narrow, 

 faint, red border, the latter attributable, I think, to the imperfection 

 of my apparatus, for my little room was full of white light and even 

 sunbeams, which were also copiously reflected, to the prism from 

 the sheet of white paper on the floor. 



After all tliis. some of your readers may ask " what is ' your 

 point'?" I will, therefore, conclude by answering (1) that 1 

 believe glass of ordinary density, and of considerable thickness, 

 absorbs the least refrangible (or red) rays of white light which has 

 already been refracted through a lens; and (2) that Sir David 

 Brewster was not far wrong when he asserted that white light is 

 composed of three primary colours, red, blue, and yellow ; from 

 which all other colours can bo produced. 



(3/<i;/ 7, 18S2).— Since the above was written, last Easter Suntlay, 

 having been to Germany, li'' llolhinil, to place my two little daugh- 

 ters at school in Cobleiitz, I took the opportunity of showing, or 

 communicating the "Green-Beam Phenomenon" to Professors 

 H. A. Lorentz^ of Leyden University; M. Kekerle, of Bonn Tni- 

 versity ; and Bruno Kerl, of the Imperial Mining .\cadcniy, Berlin, 

 who has published articles on my new system of Blowpipe Analysis 

 in the lieij nnd }Iultenmnnt\i.iche Zfi'fuii;;. 



Dr. H. A. Lorcntz, to whom I showed the phenomenon at Amster- 

 dam on Saturday, .\pril 29, was astonished that such a startling 

 phenomenon as the " Green-Beam" had not been discovered before, 

 but expressed still greater surprise when T told him I had been 

 trying in vain to get an account of it jniblished in the proceedings 

 of our .scientific societies for six months ! 



I have now made the following additional experiments : — 



8. I have passed the Green-Beam through a thin gla-ss flask filled 

 with sulphate of quinine solution, and rice rei-8'l (to sec if it was 

 due to Huorescencc). 



9. I passed it into a cobalt-bloe glass paper-weight at least two 

 inches thick. 



10. Into a thin glass flask containing a. blue solntion of ammonia- 

 sulphate of copper, and rice rei*s<*.. 



11. Reflected the focused beam from a small mirror upwjirds into 

 a thick glass paper-weight. 



12. Passed the Green-Beam through one solid glass paper-weight 

 into a prism. 



I must, however, reserve the results of these experiments for 

 another paper. 



After the above article was in type. Prof. G. G. Stokes was so 

 kind as to examine the " (ireen Beam" at the Boyal Society's 

 rooms, on May II (although there were only glimpses of sunlight), 

 and to refer me to Sir David Brewster's papers " On the Decom- 

 jiosition and Dispersion of Light," in the Philosophical JIaaazine, 

 ic, which, he says, must be determined to contain the first dis- 

 covery of this phenomenon, termed " Internal Dispersion," by 

 Brewster. Here are Sir David's own words {I'hil. Mag., 1818, 

 Vol. X.XXII., page -103) i— " In order to observe the phenomena of 

 dispersion most distinctly, I transmit a condensed beam of the sun's 

 light through the specimen when partially covered with black wax 

 or velvet. . . . page 404. I have found several glasses which possess 

 internal dispersion— one in particular, of a yellow colour, which 

 disperses a brilliant green light. ... In these cases the glass has a 

 decided colour of its own ; but I have found many specimens, both 

 of colourless plate and colourless flint glass, which disperse a 

 beautiful green light." 



Some of these words most decidedly describe in part the pheno- 

 menon I thought I had first discovered. But there is also a great 

 difference. Sir David evidently only experimented on (compara- 

 tively) thin plates of glass (those were not the days of massive glass 

 paper-weights) ; whereas the " Green Beam" can only be properly 

 exhibited in blocks or cubes of solid glass some 2i inches thick. 

 Secondly, it is cviilent from his drawings and descriptions that he 

 merely passeil the sunbeam thioigh the thin glass, whereas I 

 focussed it upon the surface of the thick. In the first case there is 

 no green beam at all, but only a kind of diffused green light, muth 

 less brilliant than the beam. 



That the plien mien- n of the green beam is not the result of 

 dispersion but of transmission, seems to me provable by the facility 

 with which it burns black cloth when passed through a crystal 

 paper-weight 21 inches thick. Who ever heard of burning chi h 

 with the green colour dispersed from a solar beam by a prism ? 

 Finally, I think it is also provable that the green beam is 



due to the union of blue and yellow rays, by the fact that if the 

 thick cubic crystal paper-weight is covered by a plato of yellow or 

 orange glass— which cuts off the blue rays-you have no longer a, 

 gro3n loam by focussing on the surface, hut only a faint yellow 

 one; whereas the green beam is invariably transmitted througr> 

 violet, pink, and other coloured glasses, which are not opaque to 

 blue ravs, with the exception of red ghiss.* through which no bean, 

 at all is transmitted into the paperweight ; i.e., white light sci'iim 

 to be re-formed. ^ 



MEASUREMENT OF THE FOCAL LENGTH OF 



DEEP CONVEX LENSES. 



Bv T. W. Webb. 



TUE difficulty of ascertaining the focal length of small convex 

 lenses within reasonable limits of error is so considerable, and 

 the result is often so untrustworthy, that 1 am induced to ask tlie 

 editor's sanction for the appearance of the follo^ving extract from oim 

 of my old note-books. Its value, 1 fear, is not in proportioii to it» 

 length, on account of the needless minuteness of its detail ; a I 

 that is worth preserving might have been compressed within miicU 

 narrower limits; but, possibly, those who, like myself, are fond of 

 manufacturing optical contrivances with ordinary materials may 

 find in their own experience some apology for its tedionsness.^^^^ 



Account of a ifethod of finding the Focal Length of a very smM 

 Lens, employed by me, y'ovem,ber 24, 1835. 



As this process may be useful, on many occasions, both to myself 

 and others, I intend to give an account of all the steps of it. 1 In- 

 deepest eve-piece of my 5.i-feet achromatic is marked by Jlr. 

 Tulley 250. To increase the power I had frequently unscrewed the 

 field-glass, and used that next the eye only ; and my object was to 

 find the power thus obtained by measuring the focal length ot th.- 

 lens. There were, however, considerable obstacles in the way. It 

 is very difticult to measure the focus by receiving the imajge ot the 

 sun on paper, because the spherical aberration of a deep 'ens >s so 

 considerable that it is hard to say where the image is best dchnetl ; 

 and this uncertainty is greater than might be imagined by a person 

 who has never madi the trial. I attempted to measure the imago 

 of the object-glass formed bv this eye-lens with a micrometer, upon 

 the principle of the dynamometer ; bvit this image was formed so 

 dose to the eye-lens that the hairs of the micrometer could not b.- 

 brought near it. It then occurred to me that if I coulii 

 ascertain the distance Letween the conjugate foci when they 

 were equidistant from the lens, one fourth of that distance 

 (by Prop XLV. of "Wood's Optics") would be the (principal) 

 focal length. To attain this object by the simplest means, 

 I took a knitting needle, and having bored holes through three 

 thick discs ot cork, made them to tlide upon it. To the centic 

 disc was glued a piece of card, into a hole in which I stuck the 

 brass cell containing the lens. In each of the other discs I stuck a 

 piece of a broken sewing-needle, so that its end might range at 

 about the same height with the centre of the lens. This apparal..R 

 is represented in the following sketch : — 



B B -6 



A is the knitting-needle, B B B the cork discs, C the card into 

 which was stuck the brass cell car^'ing the lens D, E and F tlic^ 

 pieces of sewing-needles. Tlien, by the above-mentioned pro- 

 position, if E and F could be so arranged with respect to D, that. 

 while tbev were equidistant from it, the image of E should be 

 formed at'F, and of F at K, then the distance from E to F wonl.i 

 be four times the focal length for parallel rays. I therefore took :i 

 deep lens in my hand, and while I viewed F in its focus, inoved h 

 to different distances, until its inverted image was seen m conjunction 

 with F. 



If then E and F appeared to be equi distant from D, they were in 



"•The red glass I have is red only on the upper and under 

 surfaces ; internally it is colourless. All my other coloured glasses 

 are homogeneous. 



