Intelligence and Miscellaneous Articles, 79 



tion iu their entire development, or by measuring the luminous in- 

 tensity along the whole extent of the spectrum in order afterwards 

 to deduce therefrom the corresponding coefficients of absorption. 

 It suffices for observing directly the spectral curves of equal ab- 

 sorption *, to give to the absorbent the form of a prism (or, more 

 strictly speaking, of a wedge) more or less acute, which is appKed 

 by one of its plane faces close to the slit of a spectroscope. The 

 edge of this prism is placed at one end of the slit, parallel to its 

 M 7 idth ; and we find that we thus have, over the entire length of 

 the opening, an absorbent medium, the thickness of which increases 

 uniformly from zero (at the edge) up to a certain maximum de- 

 pending on the angle of the prism and the length of the slit. 



The deviation due to the prismatic form of the medium is to be 

 destroyed by opposing to the first a second pi .ism of a material as 

 little absorptive as possible (rock-salt, fluor-spar, quartz, glass, 

 water, alcohol, &c). The angle of this second prism is easily cal- 

 culated approximately, which is sufficient in most cases ; but it can 

 at need be rendered variable within limits sufficiently wide for 

 giving at all times almost perfect compensation. 



When in this way an absorbent medium of variable thickness is 

 placed in front of the slit of a spectroscope illuminated with per- 

 fectly white light (that of incandescent solids), the spectrum no 

 longer appears, as usually, uniformly luminous throughout its 

 height, but in it shades are distinguished more or less undulated or 

 toothed, which exhibit immediately to the eye the law according to 

 which the absorption-coefficient of the medium varies with the 

 wave-length of the incident light. 



These curves can be constructed by drawing them with the 

 camera lucida, by fixing them by photography, or by referring them 

 to two rectangular axes by aid of two luminous micrometers seen 

 by reflection — one fixed parallel to the length of the spectrum, the 

 other movable and normal to the first. All these means, however, 

 of constructing the curves of chromatic absorption suppose that it 

 is possible to recognize in them the points of equal intensity, which 

 is not very easy ; but it is useful to have recourse to them to re- 

 present the complete form of the law of absorption when we have 

 to do with sufficiently absorbent substances and when rigorously 

 exact measurements are not indispensable. 



If the slit be divided into two parts in the direction of its length, 

 and each of the two halves be employed for producing a spectrum 

 with curves of chromatic absorption, the two spectra being juxta- 

 posed in the direction of their length, nothing will be easier than 

 to compare their curves and to ascertain the equality or the differ- 

 ences between them. We might even, by a tolerably simple arti- 

 fice, slide one of two spectra, of one and the same absorbent mate- 

 rial at two different degrees of concentration, over the other, 

 ascertain the zones of equal intensity, and thus apply the spectro- 

 scope to the proportioning of such substance. 



* " Metodo per determinare le curve spettrali d'assorbimento della luce 

 nei varii mezzi," di Gilberto Govi (Notizia storica dei lavori, ecc, deWAc- 

 cademia di Torino negli anni 1864 e 1865 ; adunaiiza dell' 8 maggio 1864). 



