356 Br. Herapath on the Process for 



this last portion of the beam ; it is doubtful whether the effect 

 of periodizing- the plate is lasting — I think not. 



When it is absolutely necessary to obtain a perfectly black 

 field with a total stoppage of all the incident rays (upon "cross- 

 ing" the two ciystals), it is much the better plan to employ a 

 thicker plate of this substance : such a crystal will be generally 

 found in the flask at the bottom of the mother-fluid. There is 

 more trouble requisite in obtaining perfect plates, free from all 

 intervening crystals, but the experimenter is generally repaid in 

 the end by the perfection of the polarizing medium. 



When the selenite stage is employed, the thinner and violet- 

 coloured crystals are far preferable to those which give a black 

 tone to the field ; as the colovu's are more brilliant, and the flood 

 of transmitted light much greater, so that we are enabled to use 

 a less illuminating power. I am not in the habit of using an 

 achromatic condenser with my polarizing apparatus, which pro- 

 bably accounts for some discrepancies in the results of observa- 

 tions made by different experimenters upon the same crystalline 

 plates : those crystals which will transmit the violet rays, when 

 strongly illuminated by gas or lamp light, will not do so when 

 the instrument is used in daylight, or with a plane instead of a 

 concave mirror, and without the achromatic condenser. 



If it be necessary to obtain a most decidedly black field, the 

 violet rays may be readily absorbed by interposing a thin plate 

 of sulphate of copper beneath the polarizing plate of Herapathite 

 and the source of illuminating power. 



I have recently employed a plate of this substance, one-twen- 

 tieth of an inch thick, cut on a hone, polished and mounted 

 between two plates of thin glass in Canada balsam, as a means 

 of correcting the defects of the thinner plates of these new tour- 

 malines* — this substance possessing the power of absorbing the 

 violet rays of the spectrum in a pre-eminent degree. In order 

 to succeed in this experiment, it is necessary that the sulphate 

 of copper should be inclined at a certain angle to the plane of 

 primitive polarization, as it is a substance possessing two neutral 

 axes, or planes of no-depolarizing power, the position of which 

 may be easily found, and their direction marked upon the sup- 

 port, so that the intervening plate may be always inserted at the 

 angle of its greatest activity. Professor Stokes has lately, in a 

 letter to me, suggested the employment of a glass laden with 

 the oxide of copper as a means of attaining the same end. 

 Having, therefore, prepared a boracic glass, coloured by the 

 black oxide of copper, I have used it effectually as an absorbent 

 medium for counteracting the violet-red colour of the polarized 



* A solution of the sulphate or nitrate of copper in water will equally 

 succeed in producing a black fiekl. 



