206 Professor Airy on the 



(such as I believe to exist in it) would operate in different ways 

 with right-handed and left-handed plates: and it is on this ac- 

 count that I have given both. No error is to be apprehended 

 from the jogging of the circle carrying the rhomb, as it is pro- 

 vided with four verniers, at 90° apart, all which were read. If 

 this point should be made out, it may be only a consequence of 

 the separation of the rays within the quartz: or it may be 

 another anomaly to be added to the already sufficiently compli- 

 cated phenomena of quartz. At all events, regarding the perfect 

 equality of the ellipticities as doubtful, I have carried no farther 

 the investigations made on that supposition : though the differ- 

 ence of the ellipticities is so small, that their error would be 

 insensible. 



To any one who wishes to proceed with these experiments, 

 the following hints may be of some use. 



It is convenient to have a wire carried by the rhomb, parallel 

 or perpendicular to the plane of reflection within the rhomb, and 

 in the place which I have mentioned as the position for a mi- 

 crometer: as the observer will then see that a line from the 

 center of the rings to the point where the ambiguity takes place, 

 is either parallel or perpendicular to the wire. 



The elliptically polarized light is of the same kind (always 

 right-handed) when the position of the rhomb is between and 

 90°, or between 180° and 270°: and of the same kind (always 

 left-handed) when the position of the rhomb is between 90° and 

 180°, or between 270° and 360°. 



The proportion of the axes is the tangent of the reading of 

 the rhomb-position. The major axis is parallel to that edge of 

 the end of the rhomb which makes the greatest angle with the 

 plane of reflection at the polarizing plate. 



