320 Intelligence and Miscellaneous Articles. 



that the depth of shade varies from one end to the other, a remarkable 

 phenomenon is presented by it, the law of which may be expressed 

 as follows : — Let the distances of the various points of the strip from 

 one end be laid down as abscissae, and the intensity of the light at 

 the respective points as ordinates ; then at every part of the strip at 

 which the curve formed by joining the extremities of the ordinates 

 is concave towards the axis of abscissae the strip appears lighter than 

 it is in reality, and wherever the curve is convex the strip appears 

 darker. The sensation of light, d, produced on any portion of the 

 retina by an illumination of the intensity i, may accordingly be thus 

 expressed, 



**<H*SJ} 



where x is the distance of the point in question measured longitudi- 

 nally along the strip, and f Y {z) is to be taken as representing a 

 direct function of z, and a function of the same denomination as z. 



With the help of this law it is easy to produce such a distribution 

 of the illumination that a surface which is objectively lighter shall 

 appear darker, while a neighbouring surface which is objectively 

 darker shall appear lighter. 



III. Experiments analogous to those mentioned under I. were 

 made with intermittent sounds, but the results were not such as can 

 be briefly stated. — Sitzungsber. d. K.-k. Akad. d.Wissensch. inWien, 

 1865, No. xix. p. 123. 



INVERSION OF THE ABSORPTION-BANDS IN THE SPECTRUM OF 

 ERBIUM. BY PROF. BAHR OF UPSALA. 



I have succeeded in a very simple manner in inverting the absorp- 

 tion-spectrum of erbia (and of terbia?), and in changing the ab- 

 sorption-bands into bright lines, which, as regards brilliancy and 

 distinctness, are scarcely inferior to the lines of a gas-spectrum. 



If the bent end of a fine platinum wire is dipped into a syrupy 

 solution of the nitrate or hydrochlorate of this earth, and the adhe- 

 rent salt brought into the flame of a Bunsen's burner, a spongy mass 

 of erbia is formed which shines with an intense greenish light, and 

 still greater lustre than zirconia. Placed before the slit of the spectro- 

 scope, it gives a continuous spectrum in which bright lines appear. 

 These correspond, with the exception of a few, with the absorption- 

 bands which the solutions of erbia give. 



I have not succeded in obtaining in the same way such a spectrum 

 with oxide of didymium, — which appears to prove that the dark ab- 

 sorption-band which erbia-solutions give near the Fraunhofer line D 

 does not arise, as we might be tempted to believe, from an admix- 

 ture of didymium. — Liebig's Annalen, September 1865. 



