1 60 Intelligence and Miscellaneous Articles. 



plate the face of which is exactly behind the eyepiece, so as to take 

 the impression of the phenomenon which is produced in its plane. 



By this arrangement the chemical region which is to be examined 

 can be easily put in position, although it is invisible. It is merely 

 necessary to produce in the plane of the eyepiece a sharp image of 

 the extreme limit of the luminous spectrum, the ray H for instance ; 

 the eyepiece is then removed a little so as to bring the point towards 

 the more refrangible parts, and an experiment is made by replacing 

 the ordinary eyepiece by the photographic eyepiece. The examina- 

 tion of the proof soon shows in what direction the cross wire is to 

 be moved. The want of achromatism of the lenses gives precision 

 only within very narrow limits ; and as the energy of the action differs 

 in different parts, the time of exposure ought to differ also. Hence 

 the experiments must be multiplied, and no less than eight trials are 

 necessary for producing the chemical spectrum quite complete. 



It is readily conceivable that great delicacy of detail can be ob- 

 tained; for the rays concentrated on a very small surface always retain 

 a sufficiently energetic action, however large the slit. I used collo- 

 dion sufficiently sensitive to give an ordinary photograph in five or 

 six seconds ; and the time of exposure never exceeded a minute and 

 a half. The proofs taken can be placed in a solar microscope and 

 enlarged positives obtained, but the results are imperfect from the 

 limited space which is in position ; it is better to examine them in 

 the microscope, and measure the distances of the rays with a micro- 

 metric screw placed on the stage, and draw them carefully : I have 

 obtained my drawing in this manner. The distances are not quite 

 proportionate to the deviations, owing to the slight variations of 

 enlargement ; I endeavoured especially to obtain the general aspect, 

 the form of each group, and the relative intensity of the rays. 



Many physicists, especially MM. Becquerel, Stokes, and Esselbach, 

 have already worked at this question, and designated by letters the 

 groups of the principal rays. Their nomenclature does not always 

 agree; the drawings are sometimes so imperfect as to make it dif- 

 ficult to recognize them ; and the names have been given sometimes 

 to the bright and sometimes to the dark spaces of the spectrum. I 

 have taken as guide the plate published by Miiller in his Lehrbuch 

 der Physik, applying each letter to the most remarkable obscure ray 

 of the group it served to designate. 



To illustrate the precision attainable, it may be remarked that 

 the luminous spectrum described by Fraunhofer comprises 320 rays 

 from A to H, and that in an almost equal angular space from H to I 

 I could indicate more than 280 : the results are thus comparable 

 to those obtained with light. The dispersion can be increased by 

 multiplying the prisms, as Kirchhoff has done in examining the lu- 

 minous chemical rays of coloured flame and compared them with the 

 dark rays of the solar spectrum. I propose to resume this subject 

 when circumstances permit. — Comptes Rendus, November 9, 1863. 



MAILS 



