Photo-chemistry / by S.E. Sheppard.

  • Samuel Edward Sheppard
Date:
1914
Catalogue details

Licence: In copyright

Credit: Photo-chemistry / by S.E. Sheppard. Source: Wellcome Collection.

    56/484 page 40
    the better contiguity of the images, added the Hiifner-Albrecht rhomb. § 20. Polarization Spectrophotometers. In these the diminution of the intensity is brought about by some polarizing adjustment in certain instruments, adjust- ment is made to equal visual brightness, and in others this is judged by the disappearance of interference fringes. The latter method is theoretically the more sensitive, but as Kayser remarks, involves more strain on the observer and hence fails in the promised increase of accuracy. Of the first class we have— Gian’s Photometer.1—In this the collimator slit is divided into an upper and lower half by a metal strip some 2 mm. broad. Adjustable racking of the collimator makes this form a fine dividing line between the two spectra, but the adjust- ment is different for each region. Gouy2 makes the strip wedge-shaped and slides it along till the right width inter- venes. The light from the slit, after traversing the collimator objective, meets a Wollaston prism, the principal section of which is parallel to the slit, so that of each slit image an ordinary and an extraordinary ray are formed. The collimator adjustment is made so that the upper edge of the image of the metal strip in the extraordinary ray exactly coincides with the lower, edge of the same image in the ordinary ray. Hence, meeting at the bounding line we have two spectra, their vibra- tions being polarized at right angles to each other. The alteration of intensity is effected by a Nicol prism, adjustable with a divided circle. If the Nicol is rotated from its zero point through the angle a, then the ratio of the intensities is given by I = Ift tan'-’ a. It is necessary, however, to make a series of special adjustments to obtain the actual transparency of any substance. 1 P. Gian, Wed. Ann., 1, 351 (1S87); 3, 54 (1878). 2 Ann. de Ckim. ct Phys., [5] 18, 5 (1879).
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