1054 SUMMARY OF CURRENT RESEARCHES RELATING TO 



absorption from tho radiation of the flame. The handle, being of 

 brass, obviates tho danger from breakage incident to glass handles. 



If a larger flame is desired, a 

 Fig. 224. larger burner and chimney 



muy be readily applied. The 

 adjustable stand (fig. 224), 

 since issued,* enables the lamp 

 to be set and clamped at any 

 height. The lamp may also 

 be inclined by means of the 

 horizontal axis, so as to throw 

 the beam of light downward 

 or upward, as may be neces- 

 sary, and there clamped. 



Thompson's Modification 

 of the Nicol Prism, giving 

 wider angle of field.f — In the 

 ordinary Nicol prism the 

 available polarized field is 

 limited, on the one side by 

 the intrusion of the ordinary 

 ray, and on the other by the 

 vanishing of the extraordinary 

 ray by total reflection. Of 

 the various methods suggested 

 from time to time for widening 

 the available angular aperture, 

 some have affected one side of 

 the field, some the other, some 

 bolh. For example, the sug- 

 gestion made by Prof. S. P. 

 Thompson in 1881 (and by 

 Mr. Glazebrook in 1882) to 

 alter the prism in such a way 

 as to make the balsam-film a 

 principal plane of section, has the effect (irrespective of the external 

 shape of the prism, which we may suppose given) of throwing back 

 to its furthest possible limit (for any given cement) the point at which 

 the extraordinary ray vanishes by total reflection. The obliquity of 

 the end-face, other things being given, affects the limit of intrusion 

 of the ordinary ray to a much greater degree than it affects the extra- 

 ordinary ray, hence by increasing the slope of the end-faces we may 

 add to the available width of field, but this involves increased dis- 

 tortion of the field as well as loss of light. The use of a more highly 

 refringent cement than Canada balsam causes a gain on the side of 

 the extraordinary ray — it thrusts the blue iris further back — but 



* Micr. Bulletin (Queen's), iii. (1S8G) p. 35 (1 fig.), 

 t Phil. Mag., 1SSG, pp. 478-80 (.1 pi.). 



