F. B. Pitcher — Absorption Spectra of Blue Solutions. 333 



to and entered the lower half of the slit, after total reflection 

 in a pair of small right-angled prisms. At a somewhat greater 

 distance to the left of the slit, was placed a similar Argand 

 bnrner without a condensing lens ; its rays entering the upper 

 half of the slit through another pair of reflection prisms. 

 Between this second flame and the slit was a cell with faces of 

 plane-parallel glass, within which were placed the solutions to 

 be studied. The layer of liquid thus interposed was l cm in 

 thickness. Figure 1 shows the arrangement of these four re- 

 flection prisms. In this excellent device, which 

 is due in part to Crova, the edges of the inner 

 prisms are in contact with each other and with 

 the slit. The spectra of the two sets of rays, 

 thus introduced into the collimator tube, appear 

 therefore with sharply defined adjacent bound- 

 aries, separated only by a black line. Corre- 

 sponding wave-lengths are in the same vertical 

 plane and it only remains to isolate in turn the 

 various regions to be compared, by means of an 

 adjustable diaphragm in the eye-piece, and to 

 bring the two spectra to equal intensity, in each 

 region successively, by means of the Nicol's 

 prisms. 



In its complete form, this type of spectro- 

 photometer should have two sets of Eicol's 

 prisms. One pair in the path of each ray, the 

 inner Nicol of each pair being fixed with its plane of polari- 

 zation parallel to the slit. This arrangement is advisable when- 

 ever the brilliancy of the ray under investigation is considera- 

 ble ; and it becomes necessary whenever in any region the 

 intensity exceeds that of the corresponding portion of the 

 comparison spectrum. 



The instrument in such a case possesses the advantage of 

 complete symmetry. The two rays are reflected the same 

 number of times and at the same angles. They enter the col- 

 limator tube polarized in the same plane, and throughout their 

 entire passage to the eye they suffer precisely similar treatment. 



There are, however, a great many cases, notably in the study 

 of the light reflected by pigments, and of absorption spectra, 

 where the transmitted ray suffers marked diminution, in which 

 the loss by polarization is such as to render advisable the re- 

 moval of the second pair of Nicol's prisms. This may be 

 done, whenever it becomes necessary, in order to obtain the 

 requisite brightness of all parts of the spectrum, as in the case 

 of the measurements now under consideration, without sensibly 

 vitiating" the accuracy of the determinations. 



The instrument having been arranged in the manner already 

 described ; the glass cell was placed in the path of the rays 



