504 Mr. W, N. Hartley on a new Chromic Oxalate. [Nov. 27, 



Sorby's scale. The green rays passing through a short axis consisted of 

 the red band 0-2, and one green band between 5 and 6*5, the blue rays 

 passing at right angles to the former being situated between 6*5 and 9. 

 Very frequently the blue rays were associated with the red band 0-2. 

 If these spectra be combined, the absorption-spectrum common to the 

 solution and to thin crystals is produced. 



Examined with a Nicol's prism and with similar crystals used as analyzers, 

 it was found that this substance is a powerful polarizer. There is a 

 marked difference when the blue and the green rays are examined with 

 a Mcol's prism, the former being much more completely polarized than 

 the latter, so that in rotating the Nicol there is an alternation of brilliant 

 blue and perfect darkness — if the crystal be thin, of blue and reddish 

 green. If, however, a crystal transmitting green rays of equal brilliancy 

 be examined, the light is not cut off, but only obscured. In like manner, 

 if both analyzer and polarizer consist of these crystals, when in a position 

 at right angles to each other, the effect of course is the same as that pro- 

 duced by a Mcol's prism, viz. only green rays are transmitted. The green 

 ray is more strongly refracted, and at the same time more absorbed than 

 the blue, in accordance with Babinet's law (Pogg. 'Annalen,' vol. xli. 

 p. 116). This was made evident from their apparent change of colour 

 when white light was admitted to them at different angles. These 

 observations account for the facts already noticed — namely that minute 

 crystals appear green even under the microscope, and also that the 

 powder is green. 



Let us take haphazard a number of crystals, or fragments of crystals, 

 lying in all directions over each other, and it is easy to see that no blue 

 rays can be transmitted ; for where two crystals cross, the light trans- 

 mitted is green. The green rays being more absorbed (that is to say, 

 travelling with less facility) than the blue, it is possible to have a 

 blue crystal so thin as to transmit white light through one axis, while 

 green rays would pass at right angles through the other ; under these 

 circumstances the crystals would appear green. The red colour seen by 

 gaslight is of course due to the suppression of the green and blue by the 

 yellow illumination. 



I find that Sir David Brewster*, and also Haidinger, have examined 

 the blue potassium chromic oxalate of Gregory; and many of its pro- 

 perties are those also of my salt. Brewster f states : — " In the smaller 

 crystals, which are geuerally the best formed, the colour of both reflected 

 and transmitted daylight is blue, but that of candle-light is purplish." 

 " The relations of this salt to plain and polarized light may be readily 

 examined and finely exhibited by placing upon a plate of glass a few drops 

 of a saturated solution of it in water. If the crystals are slowly formed, 

 they will be found of various thicknesses, each thickness exhibiting a 

 different colour, varying from perfect transparency through all shades of 

 * Phil. Trans, 1835, part 1. t Phil. Mag. vol. vii. p. 435. 



