478 



SCIENCE. 



[N. S. Vol. III. No. 65. 



DISCUSSION AND COBSESPONDENCE. 



EXPERIMENTS SHOWING THAT THE EONTGEN 



RAYS CANNOT BE POLARIZED BY 



DOUBLY REFRACTING MEDIA. 



To THE Editor of Science : I have, to-day, 

 made experiments which conclusively show 

 that the Eontgen rays cannot be polarized by 

 doubly refracting substances. 



On six discs of glass, 0.15 mm. thick and 25 

 mm. in diameter, were placed very thin plates 

 of Herapath's iodo-sulphate of quinine. The 

 axes of these crystals crossed one another at vari- 

 ous angles. When the axes of two plates were 

 crossed at right angle no light was transmitted; 

 the overlapping surfaces of the plates appear- 

 ing black. If the Rontgen rays be polarizable, 

 the Herapath crystals, crossed at right angles, 

 should act as lead and not allow any of the 

 Rontgen rays to be transmitted. 



On the screen covering the photographic 

 plate were cemented the six glass discs carry- 

 ing the Herapath crystals; also, three discs of 

 glass overlapping so that the Rontgen rays had 

 to pass through 1, 2 and 3 thicknesses of the 

 glass. The screening of these glasses served as 

 standards with which to compare the action of 

 the rays which had passed through one thick- 

 ness of glass and the Herapathites. On the 

 screen was also placed a square of yellow blot- 

 ting paper, J mm. thick, on which were placed 

 Herapath crystals. 



The screen of compressed brown paper was 

 impervious to two hours' exposure to a power- 

 ful electric arc light. 



On exposing the screen with the six discs 

 and paper square to the Rontgen rays, in three 

 experiments, for J hour, 1 hour and for 2J 

 hours, and developing, no traces whatever could 

 be detected of the Herapath crystals on the 

 photographs of the glass discs or on that of the 

 paper square. The contour of the paper was 

 just visible, only by very careful scrutiny. The 

 photographs of the glass discs carrying the 

 Herapathites were .circles of uniform illumina- 

 tion ; not the least mottling could be detected. 

 Through a magnifying glass these circles ap- 

 peared with a uniform grain exactly like, in 

 illumination and grain, the photograph of the 

 glass disc having nothing on its surface. 



The thinness of these crystals, their powerful 



polarizing property compared with their thick- 

 ness, and their low density of 1.8 are the reas- 

 ons why they do not at all screen (unlike cal- 

 cite and tourmaline), the Rontgen rays. These 

 well-known facts induced me to make these 

 experiments on Herapathites. They have con- 

 firmed in a very satisfactory manner what 

 Rontgen has shown by his experiments, viz., 

 that the X-rays are not polarized by their pas- 

 sage through doubly refracting media. 



Alfred M. Mayer. 



COLOR vision and LIGHT. 



In the current number of The Psychological 

 Review Mrs. C. Ladd Franklin has written some 

 very appreciative words regarding my article 

 on 'Vision' in the new edition of Johnson's 

 Cyclopaedia, but takes exception in very consid- 

 erate terms to two points which may be worth 

 a moment's attention. The first is to the state- 

 ment that the retinal cones are sensitive to vari- 

 ations of color chiefly. This was written in 

 connection with an enumeration of certain 

 optical defects common to all eyes; and, of 

 course, there was no intention to imply that the 

 cones are insensitive to that combination of 

 color variations which produces the sensation 

 of white light. Indeed, a previous sentence on 

 the same page may be found which does away 

 with all uncertainty. Nevertheless, the word 

 ' specially ' may very appropriately be substi- 

 tuted for ' chiefly. ' 



The second point is of more importance — a 

 protest against the implication that physicists 

 are satisfied with Helmholtz's theory of vision. 

 My statement that ' ' this theory, with slight 

 modification, is now quite generally accepted 

 by physicists," does not assert that they are 

 necessarily quite satisfied with it. Our opinions 

 are confessedly tentative in proportion to the 

 difiiculty of settling the matter by crucial ex- 

 periments. It is safe to say that no physicist 

 expresses his view upon this subject with any 

 approximation to the confidence with which he 

 asserts the truth of Ohm's law in regard to 

 electric currents. He is compelled to base his 

 statement upon authority ; for, as Mrs. Frank- 

 lin very rightly says, ' ' the physicists have 

 nothing to do with a theory as to what goes on 

 in the retina and in the brain." The practical 



