124 ANNUAL OF SCIENTIFIC DISCOVERT. 



COMFLEMENTARY COLORS. 



The production, by M. Niepce St. Victor, of black in photogra- 

 phy, by means of couiplomL-ntary colors, has j^iven rise to re- 

 searches on the suljjcet by !M. Chevreul, -wlio found that, although 

 complementary radiations of the spectrum produce white, those 

 radiations which emanate from complementary coloring matters, 

 ajiplied in succession or simultaneously to Ihe cloth, etc., afford, 

 according to the accuracy of the proportions, black, l)rown, or 

 gray. Thus, a blue pattern printed on orange will ajjpear black. 

 This subject, when fully developed, may have a most important 

 beai'ing on arts and manufactures. 



NEW rOLARIZIXG FRISM. 



!M^r. Ilartnack and Prazmowski recommend deviating from 

 the form of tiie Xichol prisms. The shape they recommend is 

 shoi'ter, ami has both ends normal to the incident and emergent 

 rays. According to the cementing substance employed, they give 

 the following angles : With Canada balsam, refracting index 

 1549, the faces of the Iceland si)ar make with the i)lane of sec- 

 tion an angle of 79°; with balsam of copaiba, index refr. 1.507, 

 the angle is 76°. 5 ; with linseed oil, index refr. 1485, the angle is 

 73°.5; rich poppy oil, index refr. 1403, 71°. 1. The two middle 

 ones give the largest angle of the field, viz., 35°. 



WHY THE SKY IS BLUE. 



It is generally supposed that the Idue color of the sky is due to 

 moisture in our atmosjihere ; and the idisa seems to be confirmed 

 by the intensity of the color during the moist weather of summer, 

 when compared with the sky of the more dry-weathered winter. 

 It has recently been shown by Prof. Cooke, of Cambridge, in a 

 paper read to the Amei'ican Academy of Arts and Sciences, that 

 this view is coiTCct. He has found, by means of the spectroscope, 

 — a very delicate instrument of analysis, by which the; most minute 

 substances, even when at a distance, can be detected, — that the 

 aqueous vapor of the atmosphere al^sorbs most powerfully the yel- 

 low and red rays emanating from the sun, leaving the blue rays to 

 be ti'ansmitted, and thus accounting for the color of the sky. The 

 instrument also proves that the color is due to simple absorption 

 of these rays by the water, and not to repeated reflections from 

 the surface of an infinitj^ of drops, as has been supposed. 



DEFECT IN THE POLARISCOPE, WITH A SIMPLE AND EFFECT- 

 IVE REMEDY. 



The author stated, that, having been engaged in some exjieri- 

 ments with polarized light, projected on a screen by means of the 

 oxy-hydrogen lantern, he discovered that even the best instru- 

 ments which were consti-ucted were inefiicient, inasmuch as none 

 but the axial rays transmitted through the condensers were polar- 

 ized, the main body of the luminous cone undergoing reflection 



