718 SUMMARY OF CURRENT RESEARCHES RELATING TO 



firmly retain a slide B by whicli the lung P is pressed flat. If high 

 powers are used, the wax supports must be closer together, and cover- 

 glass used. 



Fig. 147. 



Madan's Modification of Barker's Selenite-Holder. — H. G. Madan 

 has devised a modified form of selenite-holder, which he finds preferable 

 to the holder usually made. 



" The ordinary form of Barker's selenite substage fitting is well 

 known ; three films of selenite, giving retardations of | wave, | wave, 

 and f wave, respectively, are mounted in circular brass cells, which 

 rotate in rings attached to a side-arm, and can be thrown in and out 

 of the field as required. This arrangement is perfectly effective, but 

 it has two inconveniences: (1) that no means is provided for changing 

 the films for others, such as Ackland's neutral-tint film, or Klein's 

 plate ; (2) that when the selenites are in position for use it is 

 difficult to see, and impossible to tell by feeling, the exact azimuth of 

 that direction in the crystal-film, which is usually marked by 

 opticians Pf A ; i. e. the direction in which the retardation of one of 

 the two rays behind the other is greatest, and which lies, of course, at 

 an angle of 45° with the acute bisectrix, or ' median line.' 



The arrangement described below is intended to avoid both these 

 inconveniences. 



The selenite films are mounted in cells of the shape shown in fig. 

 148 ; a groove being cut in the edge of the cell before the handle A is 

 soldered into its place. The holder is in the form of a ring, about 3-7ths 

 of which are cut away, as shown in fig. 149 ; the remaining part being 

 quite sufficient to retain the cell in position, and yet allow it to rotate 

 freely when it is ' sprung ' into its i)lace. 



Three of these holders are jointed to a side-arm, so that they can 

 be thrown in and out of use, as in the usual Barker's stage. 



The handle A is made of such a breadth as to allow the selenite 

 an angular movement in azimuth of exactly 90° ; which is, of course, 

 sufficient for all modifications which it is capable of producing in a 

 polarized ray. 



Thus the various combinations of the films are obtained by merely 

 moving the projecting handle 45° on either side of its central position, 

 motions as simple and easy as those of a signalman in throwing over 

 his levers for altering points and signals. In my own case, I have 

 mounted the films in such a position that the acute bisectrix is in a 



