502 Chronicles of Science. | July, 
with a thin transparent plate, so as to modify that part of the pencil 
of rays on the side of the violet part of the spectrum, a number ~ 
of transverse bands, alternately light and dark, appear to traverse 
it. Brewster discovered that these bands were not formed when the 
thin plate was placed on the side of the spectrum corresponding with 
the red rays. It has since been discovered that these bands may be 
produced by interposing the thin plate in other portions of the path 
of the ray, besides putting it close to the eye. Baden Powell, and 
Stokes have since studied the phenomena both experimentally and 
theoretically, and the latter physicist found that the effects were best 
produced by the partial immersion of a transparent plate in the liquid 
of a fluid prism. M. Bernard has lately studied these phenomena, 
and has arranged his apparatus in the following manner :—A ray of 
solar light passing through a narrow orifice falls on the slit of a spec- 
troscope, the defringent plate being then placed between the aperture 
admitting the light and the slit of the spectroscope, and some adjust- 
ments and arrangements are made, into the detail of which we need 
not enter. In this manner M. Bernard is enabled to obtain a very 
luminous spectrum, and he has been led by an examination of the 
phenomena to the discovery, that through them he is enabled to 
obtain the length of the waves of any desired ray of light or spectrum 
line with much greater accuracy than by the ordinary diffraction 
method. In his memoir he has given the wave lengths of the seven 
principal rays of the solar spectrum, together with that of the ray A, 
which, owing to its faintness, has not yet been satisfactorily deter- 
mined, and the green ray of thallium. Their values, expressed in 
millionths of a millimetre, are— 
A = 760-6 
352i 
The diffringent plate of quartz is about a millimetre thick, and its 
thickness can be determined with absolute accuracy with the sphero- 
meter; and when it is remembered that between A and H there are 
for this thickness more than 700 interference bands, and that it is 
easy to estimate to the tenth of a band, it is seen that there are more 
than 7,000 invariable points in this portion of the solar spectrum, and 
it is by reference to these that M. Bernard proposes to classify the 
rays of the alkaline metals and other interesting spectra. For this 
purpose he has constructed an apparatus which acts both as a spectro- 
scope and a goniometer, and which enables the observer to measure to 
within 10”, the indices, a knowledge of which is necessary to calculate 
the wave lengths. 
Heat.—Some important results have been communicated to the 
Berlin Academy by M. Hagen,} respecting the heat of the sun’s rays. 
He has come to the conclusion that the heating effect produced by 
the sun’s rays on entering this atmosphere may be expressed by say- 
* Dr. J. Miiller (¢ Quarterly Journal of Science,’ vol. i. p. 157) finds the length 
of the wave of the green thallium line to be 53848 mitlionths of a millimetre. 
t ‘Phil. Mag.,’ ser. iv. vol. xxvii. p. 478, 
