On a New Optical Instrument, &c. 139 



little ellipse in which the reflected vibration is supposed to be 

 performed. Now, the axes of this ellipse are parallel and per- 

 pendicular to the principal plane of the rhomb, when it is in 

 the situation above described, where the light completely dis- 

 appears ; and the ratio of the axes is the tangent of the angle 

 which that plane makes with the principal section of the eye- 

 piece next the eye. The angles are read off from the divided 

 circles ; and thus, for any angle of incidence, and any plane 

 of primitive polarization, we can at once ascertain the nature 

 of the reflected elliptic vibration. Professor Mac Cullagh men- 

 tioned that the instrument was made last year with the view 

 of testing certain formulae which he has proposed for the case 

 of metallic reflexion, and which have been printed in Yol. xvm. 

 pp. 70, t 71, of the Transactions of the Academy*; but that he 

 had not yet found leisure to make the various adjustments 

 which are necessary in order to obtain satisfactory results with 

 it. The instrument is beautifully executed by Mr. Grubb, who 

 himself contrived the subordinate mechanism, by which the 

 requisite movements are effected with perfect ease to the ob- 

 server. 



* Supra, pp. 132, 133. 



