May 1, 1885.] 



♦ KNOWLEDGE ♦ 



363 



prizes, or 132,500 dols., the other half being drawn for the 

 numbers held by the compftuy. Thi-! hilf is paid in full 

 like the other ; but paying it in full means liandii)<; it over 

 to the couipunv out nf their own capital. Tlie balance of 

 profit for the iuonth is thus 1 17,">00dol-'., or the net profits 

 112,000 dols., assuming the above liberal allownnoe for 

 expenses. It is probaMe that the net monthly prolits 

 never fall short of 100,.")00 Jola, or .£210, 000 per annum. 



Yet though the swindling nature of the whole transaction 

 is thus not only obvious, but, as it were, practically insisted 

 upon in the .advertisements of the company, fools rush in, 

 in mvria<is monthly, to pay their live dollars for chances 

 wortt but two dollars sixty-five cents. If a man held an 

 open lottery, to which each of one hundred persons admitted 

 paid £5, and taking the sum of £500 thus collected, were 

 to say — "The lottery, gentlemen gamblers, will now pro- 

 ceed ; £265 of the sum before me, I will distribute in 

 prizes, as follows " (indicating how it would be divided) ; 

 "the rest, the sum of £235 which I have here separated, I 

 will put into my own pocket " (suiting the action to the 

 word), " for my trouble in getting up this lottery " — it 

 strikes me the " gentlemen gamblers " would object to the 

 arrangement Yet the Louisiana lottery swindle does this 

 very thing on a larger scale month after month ; and each 

 year hundreds of thousand-", greedy to be rich without 

 labour, pour in their five dollars apiece in an endless stream. 

 And the State of Louisiana not only allows this gross 

 swindle to go on, but the lottery is a State lottery, and has 

 been " incorporated by the Legislature of Louisiana for 

 education and charitable purposes." — Newcastle Weekly 

 Chronicle. 



CRITICAL METHODS OF DETECTING 



ERRORS IN PLANE SURFACES. 



By John A. Brashear. 



(Continued from p. 348.) 



I NOW invite your attention to the method for testing 

 the flat surfaces on which Professor Rowland rules 

 the beautiful diffraction gratings now so well known over 

 the scientific world, as also other plane surfaces for Helio- 

 stats, itc. I am now approaching the borderland of what 

 may be called the abstruse in science in which I humbly 

 acknowledge it would take a vast volume to contain all I 

 don't know, yet I hope to make plain to you this most 

 beautiful and accurate method, and for fear I may forget 

 to give due credit, I will eay that I am indebted to Dr. 

 Hastings for it, with whom it was an original discovery, 

 though he told me he afterwards found it had been in use 

 by Steinheil, the celebrated optician of Munich. The 

 principle was discovered by the immortal Newton, and it 

 shows how much can be made of the ordinary phenomena 

 seen in our everyday life when placed in the hands of the 

 investigator. We have all seen the beautiful play of 

 colouri on the soap bubble, or when the drop of oil sjjreada 

 over the surface of the water. Place a lens of long curva- 

 ture on a piece of plain polished glass, and, looking at it 

 obliquely, a black central spot is seen with rings of various 

 width and colour surrounding it. If the lens is a true 

 curve, and the glass beneath it a true plane, these rings of 

 colour will be perfectly concentric and arranged in regular 

 decreasing interval^. This apparatus is known as Newton's 

 colour-glass, because he not only measured the phenomena, 

 but estaVjlished the laws of the appearances presented. I 

 will now endeavour to explain the general principle by 

 which this phenomena is utilised in the testing of plane 

 surfaces. Suppose that we place on the lower plate lenses 



of constantlj' increasing curvature until that curvature 

 becomes vil, or, in other words, a true jilane. The rings of 

 colour will constantly increase in width as the curvature of 

 the lens increases, until at last one colour alone is seen over 

 the whole surface, provided, however, the name angle of 

 observation be maintained, and provided further that the 

 film of air between the glasses is of absolutely the same 

 relative tiiickuess throughout. I say the film of air, for 1 

 presume that it would be utterly impossible to exclude par 

 tides of dust so that absolute contact could take place. 

 E irly physicists maintained that absolute molecular contact 

 was impossible, and that the central separation of Iho 

 glas.ses in Newton's experiment was 7T-g^,\,^iii of an inch, but 

 Sir William Thompson bus shown tliat the separation is 

 caused by shreds or particles of dust. However, if this 

 separation is equal throughout, we have the phenomena 

 as described, but if the dust partichis are thicker under 

 one side than the other our phenomena will change 



c 



FiR. 7. 



to broad parallel bands, as in Fig. 8, the broader the 

 bands the nearer the absolute parallelism of the plates. 

 In Fig. 7 let a and h represent the two plates we 

 are testing. Rays of white light c, falling upon the 

 upper surface of plate a, are jiaitially reflected off in the 

 direction of rays d ; but, as these rays do not concern us 

 now, I have not sketched them. Part of the light passes 

 on through the ui)per plate, where it is bent out of its course 

 somewhat, and, falling upon the lower surface of the upper 



Fig. 8. •'.:'; 



plate, some of this light is again reflected toward the eye at 

 d. As some of the light passes through the upper plate, 

 and, passing through the film of air between the plates, 

 falling on the upper surface of the lower one, this in turn is 

 reflected, but as the light that falls on this surface has had 

 to traverse the film of air twice, it is retarded by a certain 



