170 THE AMERICAN MONTHLY [June, 



tion-plates, combined with the slight separation of the 

 optic axes, rendered it impossible to measure the obtuse 

 angle in mouobromnaphthaline, and so to calculate the 

 true angle by means of the formula 



Tan Va equals Sin Ha divided by Sin Ho. 



The difficulty was surmounted, as fully described in 

 the memoir referred to, by measuring the acute angle 

 by means of section-plates perpendicular to the first me- 

 dian line immersed successfully in two liquids, benzene 

 and cedar oil, whose refractive indices were nearly, and 

 the mean of them exactly, equal to the mean refractive 

 index of rubidium sulphate. The two series of valves 

 obtained for six wave-lengths of light (the monochro- 

 matic light producer as described in Phil. Trans. 1894, 

 A 913, being employed) were almost identical, differing 

 only by a very few minutes, and the mean for each wave 

 length was taken as representing the true angle of sep- 

 aration of the optic axes for that particular wave-length. 

 The method is applicable to all cases where it is found 

 impossible to see the hyperbolic brushes through a sec- 

 tion perpendicular to the second median line on account 

 of the slight separation of the optic axes. Mr. Miers 

 has had a goniometer constructed for the express pur- 

 pose of studying the use of an immersion fluid. 



Another case in which observations in such a liquid 

 are of great value when it is found desirable to confirm, 

 in some independent manner, the mode of dispersion of 

 the optic axes for different colors indicated by the cal- 

 culated valves of 2 Va obtained from the formula last 

 quoted. Several of the compounds which the writer has 

 lately been engaged in studying exhibit very low dis- 

 persion of the optic axes, and the calculated values of 2 

 Va for five wave-lengths, obtained from the measure- 

 ments of the apparent acute and obtuse angles in mono- 

 bromnaphthaleue by the use of accurately orientated 



