162 Prof. D. B. Brace on a 



the spectrophotometer the best results have been obtained by 

 means of a silver strip. The photometer is subject to the 

 further restriction in its use of measuring the quantities of 

 light. This element, as a difficulty, is really negligible in 

 the polariscope, but the former difficulty does not seem yet to 

 have been overcome. 



In a polariscope it is necessary to vary the so-called 

 sensibility as the amount of light varies. This condition 

 should obtain too for all colours. For the greatest efficiency, 

 the polarizing and analysing elements should not displace the 

 ray when placed in its path, as this would generally affect the 

 position of the image for any one wave-length. The bounding 

 lines between the fields should also vanish for any colour and 

 sensibility and that too whether a broad or a narrow radiant 

 is the source of light. Some of these conditions have been 

 realized in the types already devised, but no one embodies 

 them all. 



In the biquartz of Soleil a vanishing line is partially 

 realizable, but as the tint of passage is used white light is 

 required ; and further the sensibility cannot be varied. In 

 a double rotary element, such as that of Poynting rotating 

 differentially in the same direction, a vanishing line is also 

 partially realizable ; but its use over a finite portion of the 

 spectrum of sufficient breadth to give a proper intensity will 

 make initial settings of different tints in the different parts of 

 the field, owing to the differential rotation. The neutral 

 position of the analyser will also vary in different parts of 

 the spectrum for similar reasons. This form and the modified 

 form of the biquartz with a small angle of rotation which is 

 used for monochromatic light do not admit of a practical 

 variation in the sensibility. Furthermore, the added rotation 

 of any medium under examination increases the differential 

 rotation of one portion of the field over the other, thus 

 intensifying the difference of tint in the two fields, which 

 diminishes the sensibility in setting. In the polariscopes 

 of Savart, Babinet, Nodot and others a vanishing and a 

 displacement of interference-bands take place. The settings 

 which can be made with these forms are far less accurate 

 than those already mentioned and in some of them very 

 fatiguing to the eye. In the system of Laurent, with a half- 

 wave plate, much used in saccharimetry at the present time, 

 a vanishing line is attainable and the sensibilitj' can be varied, 

 but light of only one colour can be used. In the half-shade 

 polarizer of Jellet any colour may be used, but it is impossible 

 to vary the sensibility and also to eliminate the dark line 

 between the two halves of the field. In the half-shade 



