6l2 



KNOWLEDGE & SCIENTIFIC NEWS. 



[December, 1906. 



niisjht he supposed to be from the way in which they 

 are often reft-rred to. They assume that the lens is 

 free from everv aberration, and even then do not in- 

 clude the effect of the obliquity of the rays that fall 

 upon the plate everywhere except centrally opposite the 

 lens. Thev are serviceable for all s^ood lenses in the 

 middle of the field, and so far as the field is flat and 

 the defining^ power of the lens dees not fall off. With 

 the larg-er apertures of a " rapid rectilinear " lens, this 

 will be a very small area of the plate that it is supposed 

 to cover; with a modern anastiijmat it will lie of greater 

 extent, but lenses of both classes vary very much in 

 qualitv. 



CORRESPONDENCE. 



A System of Applied Optics. 



To the Kditors uf " Knowledc.e iv .StiENru-u Xkws." 

 Dear .Sirs, — While feeling' highly gratified bv the kindly 

 and appreciative review of the above work which appeared 

 in your November number, I would like to point out that 

 I do not introduce an altogether new convention as to signs, 

 as your reviewer iriipiies. In fact, I have always arranged 

 my conventions in entire accordance with those adopted by 

 Coddington, in 1S29, in his own work, and which have 

 always struck me as the most practically reasonable sign 

 conventions that have yet been put forward among the 

 various writers on optics, all other methods being too 

 abstract and academical. 



Yours, etc., 



H. DENNIS T.WLOR. 

 York, November 16, 1906. 



N Rays. 



To the Editors of " K\(i\vm;dge cV S( n.:Nru-i(- News." 

 De.ar .Smjs, — I should be obliged if you could tell me 

 whether any investigators other than M. Blondlot, have ob- 

 tained indicatioEis of the supposed N radiation. What do 

 physicists in general think with regard to Blondlot 's results, 

 and his belief in his discoverv? 



r. M. RYVE.S. 

 Zaragoza. 



October ig, igoG. 



[Results partly subjective ; in so far as they arc objective 

 probably N rays are long heat-waves. General attitude 

 sceptical .— .\. W. P.]. 



A Curious Optical Effect. 



To the Editors of " Knowledge & Scientific News." 

 Dear Sirs, — The explanation of the above-mentioned by 

 " 13," in your last issue, is, I take it, comparatively simple 

 and would constitute a good " Exam." question for students 

 111 optics and sight testing. The clear perception of a line 

 at any angle depends on the relative curvature of the various 

 meridians of the eye. Generally, the cornea of the eye 

 has less curvature in the horizontal than the vertical meri- 

 dian, and, as a consequence, the diffusion patches which 

 form the total image of a line overlap on themselves in the 

 horizontal meridian, with the result that the margins of the 

 line remain clear and distinct, whilst those in the meridian 

 at right angles overlap the margins, causing them to be 

 indistinct. If the diagram is looked at with the page held 

 horizontally, a reverse effect will be obtained. It is, per- 

 haps, unnecessary to say that this defect is known as 

 astignratism, and that Sir G. .\iry was one of the first 

 persons to notice this defect of the eye, being astigmatic 

 himself. 



Yours trulv, 



W. B.ANKS. 

 Bolton. 



ASTRONOMICAL. 



By Ch.vrles P. Butler, A.R.C.Sc. (Lond.1, F.R.P.S. 



Physical Investigations of Sun Spot 

 Spectra. 



Messrs. Hale, .\dams, and Gale have recently published 

 the results of a long investigation on the causes determin- 

 ing the chief phenomena of sun-spot spectra, giving com- 

 parisons with similar variations of the spectra of terrestrial 

 substances treated in different ways in the laboratory. One 

 of the chief experiments consisted in volatilising the 

 elements most important in sun-spot spectra under two or 

 more degrees of temperature, produced by electric arcs of 

 30 and 2 amperes respectiveh', and by the electric spark. 

 Tables are given of the lines of Titanium, Vanadium, Iron, 

 Chromium, and Manganese, extending from the ultra- 

 violet to ^ 5800, showing their varying intensities when 

 subjected to these different heat sources. The main results 

 show that over ninety per cent, of the lines, which are 

 strengthened in sun-spots, are found to be strengthened in 

 [jassing from a 30-ampere arc to a 2-ampere arc. Also, 

 of the lines weakened in sun-spots, over ninety per cent, 

 are weakened or absent in the 2-ampere arc. 



Over ninety per cent, of all the enhanced, or special spark- 

 lines, are weakened or absent in the 2-ampere arc. From 

 many collateral results it is generally considered that the 

 temperature of an electric arc decreases with decreasing 

 current strength, so that the above conclusions certainly 

 point the suggestion that sun-spots are regions of lower 

 temperature than the surrounding photosphere. — (Astrij- 

 l>hyskal Journal, xxiv., p. 1S5, October, K)o6.) 



Jupiter's Seventh Satellite. 



The seventh satellite of Jupiter has been again observed 

 by Professor Perrine, at the Lick Observatory. The co- 

 ordinates of the obser\-ed position were : — 



Position angle = 119°.! 1 igo6 September 25.9962 

 Distance = 2578" J G.M.T. 



Cape of Good Hope Observatory 

 Report for 1905. 



In his report on the work of the Cape Observatory during 

 UJ05, Sir David Gill gives special prominence to the pro- 

 gress made with the new transit circle. Some trouble has 

 been experienced with the completion of the new under- 

 ground azimuth marks, but the pits are now water-tight 

 and the lenses mounted in position. Observations have 

 been regularly made with the new system since October 31, 

 1905. For the adjustment of the marks in relation to the 

 long focus collimating lenses, a new method was devised 

 to get over the difficulty of bringing a spider web into 

 coincidence with its own reflex image, and this is said to 

 give an error of only + o.oi" for a single pointing. An im- 

 portant point brought out by this method of observation is 

 the systematic diurnal variation of the azimuth of the 

 transit circle between sunset and midnight. Although the 

 discussion of the observations of circumpolar stars is not 

 yet sufficiently advanced to determine the absolute variation 

 of the azimuth marks, the opinion is expressed that they 

 may prove sufficiently stable to determine the horizontal 

 component of the Chandler change of latitude. The chief 

 drawback is the frequent apparent unsteadiness of the 

 marks, produced by irregularities of refraction in the in- 

 tervening air, and it may ultimately be desirable to protect 

 the intervening ground from the sun's rays. 



