Dr. Tyndall on the Progress of the Physical Sciences. 173 



iodide of the disulphate of quinine ; one as a polarizer, the other 

 as an analyser, the selenite and disulphate of quinine being in- 

 terposed. This will fully establish the fact of this substance 

 possessing optical properties precisely equivalent to those of the 

 tourmaline, or of a Nicolas prism, and will be sufficient to show 

 that all the phsenomena capable of being produced by the one 

 may be exhibited by the other. 



Upon submitting these artificial polarizing plates to micro- 

 metrical admeasurement, it was found that those which possessed 

 sufficient thickness to adhere together in clusters, and to raise 

 themselves on their edges so as to show their thickness, were 

 none of them more than j^dth of an inch ; many were about 

 one-half or one-third of this thickness — g-^ or y A^dth of an 

 inch. But even these were much larger than any of those thin 

 broad plates so readily broken ; and some of which, after great 

 trouble, were mounted and experimented with. The tourmalines 

 commonly sold and employed for optical purposes are from yi^dth 

 to gigdth °f an * ncn thick ; such a one as the latter size was 

 employed in the comparative experiment above related, whence 

 it follows that this newly-discovered substance possesses the 

 power of polarizing a ray of light with at least Jive times the in- 

 tensity that the best tourmaline is capable of. It must conse- 

 quently be the most powerful polarizing substance known, and 

 it has been proved to be a new salt of a vegetable alkaloid. 



32 Old Market Street, Bristol, 

 Nov. 30, 1851. 



XXVII. Reports on the Progress of the Physical Sciences. 

 By John Tyndall, Ph.D. 



On Electric Currents of the First and Higher Orders. By P. Riess, 

 Poggendorff's Annalen, vol. lxxxi. p. 428, and vol. lxxxiii. p. 309. 



WHEN an electric battery is discharged, the current which 

 passes through the connecting wire is known to be 

 capable of inducing a secondary current in another wire brought 

 near it ; and if the secondary current be permitted to operate 

 upon a third wire, a tertiary current will be induced, which in 

 its turn will induce a current of the fourth order in a fourth 

 wire, and so on. The current which passes through the wire 

 directly connected with the battery will in the following be called 

 tin- principal or primary current. The object of M. Riess appears 

 to have been to make a strict investigation of these various cur- 

 i be circumstances under which i hey appear, their influence 

 upon each other and upon themselves ; and finally, to clear up 

 the doubt which at present exists as to their directions. 



