142 ANNUAL OF SCIENTIFIC DISCOVERY. 



that heat is constantly developed with superior intensity at the positive 

 end of any voltaic arrangement. 



COMBINATION OF POLARISCOPE AND REFLECTING GONIOMETER. 



MR. W. P. BLAKE, of the New Haven Laboratory, presented to the 

 meeting- of the American Association, in August, an instrument de- 

 signed for taking the angles included between the optic axes of crys- 

 tals and for use as a reflecting goniometer. The general arrangement 

 of the instrument is similar to that of Wollaston's goniometer, but it is 

 provided with a black glass reflecting-plate for polarizing light, which 

 is also used to reflect the image of bars of the window or a string v/hen 

 measuring solid angles. The analyzing lens or eye-piece is either a 

 tourmaline or Nichols prism, and is combined with small convex lenses 

 of short focus, which are useful to enable the observer to obtain a dis- 

 tinct and clear image of the colored rings surrounding the optic axes, 

 especially when the mineral to be examined is in thin plates. The 

 mineral is held in the proper position, at right angles to the polarized 

 ray, by means of a peculiar clamper vice, which allows of such adjust- 

 ment as will bring the plate of mineral into coincidence with the axis 

 of rotation of the shaft to which it is attached. When the instrument 

 is to be used as a goniometer, this clamp is taken off from the end 

 of the shaft, and replaced by a contrivance for the adjustment of crys- 

 tals, resembling that adopted by M. Mitscherlich ; the polarizing eye- 

 piece can also be removed and a small telescope substituted, or the 

 angle may be taken without, as with the common goniometer of 

 Wbllaston. 



In order to measure the angle between the optic or resultant axes 

 of crystals, the instrument is placed near a window, so as to receive 

 the light reflected from ths clouds without interception. A string is 

 then stretched across the window space at such a height that its image 

 will be in the field of view ; it should cross the field horizontally and 

 bisect it. A .vertical string is also to be arranged so that its image 

 crosses that of the other at right angles. The Nichols prism or ana- 

 lyzing piece is then turned or " crossed'' so as to extinguish the ray 

 polarized by the black glass reflector. The plate of mica or other 

 mineral to be examined is than interposed between the polarizer and 

 eye-piece, and turned until the position of the poles are found by means 

 of the shape and position of the colored rings ; it is then to be secured 

 by means of the clamp, and if the plane in which the axes lie is found 

 to be parallel with the vertical string, it is ready for measurement. 

 on the circle being brought to on the vernier, the shaft is turned 

 so as to incline the mineral to the polarized ray, and when the elliptical 

 rings or dark spots around the pole are accurately bisected by the im- 

 age of the horizontal string, the shaft is clamped to the circle, and both 

 turned together in the opposite direction until tho rings or spots around 

 the other pole come into the same relative position ; the angle through 

 which the mineral has moved is then read off, and is the apparent incli- 

 nation of the optic axes. 



The instrument was constructed more especially for the optical ex- 



