138 On the Coloured Rings of Uniaxial and Biaxial Crystals. 



From the development of these general results it will be evi- 

 dent that they fulfil the following conditions :— ? 



I. The irregularity of signals in the one station is entirely inde- 

 pendent of the irregularity of signals in the other station. 



II. The irregularity of signals in each station is due only to 

 balance not being rigidly established. 



III. If balance in either station is disturbed, a single adjustment 

 in the branch b will reestablish that balance. 



IV. Any disturbance of balance will have the least possible 

 effect on the received signals. 



V. Maximum current at balance. 



VI. Maximum magnetic effect of the maximum current on the 

 receiving -instrument. 



[To be continued.] 



XXI. On a simple Arrangement by which the Coloured Rings of 

 Uniaxial and Biaxial Crystals may be shown in a common Mi- 

 croscope. .By Dr. W. H. Stone*. 



TiHE author was not aware that any arrangement had been 

 hitherto supplied to the ordinary microscope other than an 

 extra top to the eyepiece containing a supplementary stage and 

 an analyzer. This could only be considered a clumsy expedient. 



The objects to be obtained were clearly two : — first, to transmit 

 the rays at considerable obliquity through the plate of crystal ; 

 secondly, to gather these up and form a real image within the 

 tube of the microscope. Amici had accomplished this by a 

 special combination of lenses which bears his name; it might, 

 however, be done simply by placing a screwed diaphragm 

 on the end of the upper draw-tube within the body of the mi- 

 croscope. The screw should be that ordinarily used for object- 

 glasses. To this an object-glass of long focus was fitted, and 

 another of higher magnifying-power in the usual place. The 

 whole body was then drawn out and adjusted to a- telescopic 

 focus on a distant object. The lower objective formed the 

 object-glass of the telescope, and the inner objective with the 

 Huygenian eyepiece a compound ocular. On reinserting the 

 body thus arranged, and illuminating the crystal on the stage 

 with convergent light by means of a condenser, the rings and 

 brushes could be perfectly seen. The whole double series of 

 rings in a biaxial crystal of carbonate of lead was thus shown. 



The condenser used was a " kettle-drum " of two plano-convex 

 lenses. The objective on the nozzle of the microscope was a f- 

 of Ross; that within the draw-tube a 3-inch objective of the 

 same maker. 



* Read before the Physical Society, June 13, 1874. Communicated by 

 the Society. 



