TRANSACTIONS OP THE SECTIONS. 13 



those whose vision is in any way abnormal, and no other instance was known to the 

 late Prof. George Wilson, whose book is the standard one on the subject of colour- 

 blindness. 



On the Chromatic Properties of the Electric Light of Mercury. 

 By J. H. Gladstone, Ph.D., F.R.S. 



While examining the brilliant electric light produced in an interrupted current of 

 mercury in the apparatus contrived by Professor Way, the author was struck by the 

 strange manner in which it modified the apparent colours of surrounding objects, 

 and especially with the ghastly purple and green hues which it imparted to the faces 

 and hands of the spectators. This led him to an investigation of the subject, and a 

 prismatic analysis of the light itself. Chevreul's " cercles chromatiques " showed 

 yellow, green, and blue distinctly, but very little red, while the violet became remark- 

 ably luminous. The modifications of colour in many bodies of known composition 

 were then related, as for instance the green sulphate of iron which appeared colour- 

 less, and the scarlet iodide of mercury which assumed a brownish metallic appear- 

 ance. Substances capable of fluorescing exhibited that phenomenon with remark- 

 able beauty. On analysing this light by means of a refractive goniometer, the author 

 found it to consist of a great number of separate rays, and not to present in any 

 part a continuous band of light. This was exhibited by means of a diagram in 

 coloured chalks on black paper, by the side of a solar prismatic spectrum. The 

 position of the different rays had been measured, and their relative intensity deter- 

 mined. There are red and orange rays, but they are of the most feeble intensity ; 

 some yellow rays of great brilliancy ; two bright green rays ; one blue ray of great 

 luminosity ; and a number of violet rays. One of these latter is situated far bej'ond 

 the limits of the visible solar spectrum, in fact at about Becquerel's line N, and was 

 bright to the eye, although it had passed through several pieces of glass — a medium 

 that does not easily transmit the extra-violet rays. Its colour appeared to differ 

 considerably according to its intensity, but might be described generally as a red- 

 violet. The prismatic analysis explained fully the changes that red substances un- 

 dergo when exposed to it — sometimes to brown, and at other times to purple, green, 

 or whatever other colour in addition to red is principally reflected by them : it also 

 explained all the other chromatic phenomena. Professor Wheatstone in 1835 de- 

 scribed the spectrum of the electric light of mercury as containing seven definite rays ; 

 and Angstrom has recently given a drawing of the lines that coincides closely with 

 the observations of the author on the more luminous rays, and shows that the Swedish 

 physicist had not seen the extra violet lines. From his figures also it appears that 

 the air is excluded from the luminous cone of mercurial vapour in Way's apparatus. 



On a New Instrument for determining the Plane of Polarization. 

 By the Rev. Professor Jellett. 

 Professor Jellett described to the Section a new analysing prism, by which the 

 plane of polarization of polarized light may be determined with great precision. 

 This instrument consists of a long prism of calc-spar, which is reduced to the form 

 of a right prism by grinding off its ends, and sliced lengthwise by a plane nearly 

 but not quite perpendicular to its principal plane. The parts into which the prism 

 is thus divided are joined in reversed positions, and a diaphragm with a circular 

 opening is placed at each end. The light which passes through both diaphragms 

 produces a circular field divided by a diametral slit into two parts, in which the 

 planes of polarization are slightly inclined to one another. If then light which has 

 been previously plane polarized be transmitted, it will be extinguished in the two parts 

 of the field of view in positions which lie close together, and the light will become 

 uniform in a position midway between these. This position determines the plane in 

 which the incident light was polarized, with a precision much greater than has been 

 otherwise attained. Professor Jellett stated that the different observations did not 

 differ from one another by an angle greater than a minute, and that the instrument 

 was equally applicable to the case of homogeneous light. 



