Dr. E. J. Mills on Nitrated Toluol. 17 



which pass through the centre O of the curved surfaces of the 

 lenses ; but when a crystal is introduced, the rays may be sent 

 through it in any desired direction on turning the axis, so that 

 either of the optic axes of a biaxal crystal may be brought into 

 the centre of the field. Thus the angle between the optic axes 

 may be measured. The central piece M N is made in the form 

 of a box, so that a liquid may be introduced into it for mea- 

 surements in those cases in which the optic axes are too far apart 

 to be seen in air. 



The advantages to be obtained by the use of this central pieca 

 are: — 



(1) The extension of the field of view. If the angle in air cor- 

 responding to the field of view is 74° without the central piece, 

 then the angle will be increased to about 128° when the central 

 piece is introduced, the central piece giving the same angle in 

 glass that is given without it in air. The field of view may be 

 made to include both optic axes of topaz of Brazil. 



(2) When the plane containing the optic axes is at right 

 angles to the axis P Q, either of the optic axes of a biaxal crystal 

 or any ring may be brought into the centre of the field of view 

 where the spider-lines cross one another, and the angles between 

 them accurately measured. Instead of employing a lens for an 

 eyepiece, the centre of the field may be determined by fixing 

 crossed spider-lines at a point halfway between the lenses D 

 and E. These lines will be brought to a focus by the system of 

 lenses E, M, N, F, and G at a distance to the right of G equal to 

 about one tenth of an inch; and at this focus another set of 

 crossed spider-lines should be placed, so that the two sets may by 

 their coincidence determine the centre of the field of view. The 

 spider-lines should be in the focus of the lens H, so that they 

 may be seen on looking through the NicoPs prism. 



III. On Nitrated Toluol 

 % Edmund J. Mills, D.Sc, F.R.S* 



JUTONONITROTOLUOL.— Toluol free from benzol, and 

 having a nearly constant boiling-point, was nitrated by 

 dropping into "fuming" or nitrous nitrate of sp. gr. 1*48. The 

 product was distilled in a current of steam (an operation neces- 

 sary to purify it) ; toluol then came over first, and afterwards 

 a mixture of solid with liquid mononitrotoluol. The two iso- 

 mers can be separated with great completeness by cooling for 

 half an hour to —17°, and suddenly filtering with the aid of a 

 pump. The solid modification remains on the filter; the liquid 



* Communicated by the Author. 



Phil May. S. 4. Vol. 50. No. 328. My 1875. C 



