ILLUMINATION IN CONNEXION WITH POLARIZATION. 175 



light be taken from that zone of the condenser which will afford rays 

 of the required degree of obliquity for each occasion ;* but it must be 

 remembered that with all transparent substances it is the refracted 

 (and not the reflected ray as for opaques)^ that is utilised, and hence 

 it is that a modification of arrangement becomes necessary. 



For an opaque object the light is purest and best when taken direct 

 from its source and thrown down at the required angle by a metallic 

 reflector, without the intervention of any bullseyeor other condenser ; 

 but when the refracted ray is required, the reflected rays are dispensed 

 with, and may be sifted out to commence with. If the mirror were 

 of polished silver, or speculum metal, the light would be thrown 

 upwards undecomposed ; but with the silvered glass, a portion is first 

 reflected from its upper surface, after which the refracted portion is 

 sent up from the silvered under-surface ; now, as the reflected ray is 

 in the plane of the reflecting surface, and the refracted ray at right 

 angles to it it is obviously necessary that any subsequent reflections and 

 refractions as by the glass slip containing the object, should preserve 

 the same relative position of plane and refracting surface. It is, 

 therefore, best to place the light directly in front of the stage, and 

 at a distance suitable for admitting of the mirror being adjusted, so 

 as to receive its rays at an angle of about 5I|° (which may readily 

 be done by the help of the card scale placed across the setting of 

 the mirror) ; the refracted and the reflected rays will thus be 

 separated from each other by a distance proportionate to the thick- 

 ness of the silvered glass and the incident angle of the ray. A 

 second reflection and refraction then takes place at the under side of 

 the glass slip, and others successively from the object and from the 

 cover ; but each set of rays separated by the mirror will follow its 

 own course and behaviour, and, in all subsequent stages, producing 

 its own corresponding results. 



By thus getting rid of direct reflected rays, we at once dispose of 

 the question of contracting diaphragms and other means of lessening 



* Since this paper has been in progress, I have succeeded in adapting the 

 principle of the side reflector as a substage arrangement for transparent 

 objects, which possesses all the merits of the Brainhall mode of illumination 

 without any of its defects ; and combines also the additional advantage of 

 being applicable to general purposes (which the other is not) producing a 

 beautifully soft and clear light, with great perfection of definition. It is 

 so arranged that the angle of incidence may be regulated at will and set 

 at any degree of obliquity capable of producing from a darkground illumi- 

 nation to almost direct rays; whilst it admits of the lamp being placed in 

 front of the instrument, so as to be entirely out of the way of the hands 

 and face. 



