Mr. W. H. Walenn on Unitation. 521 



The effectiveness of a polarimeter for the purpose of observing 

 values of polarization such as occur in the sky at right angles to 

 the sun is determined, by different conditions from those required 

 for the observation of neutral points, and of negative polariza- 

 tion such as occurs in the sky. For the former purpose a very 

 sensitive test-object is not needed, but it is required to know 

 the values of the inclinations of the glass bundle. For the Jatter 

 purpose, on the other hand, it is necessary to have a test-object 

 of the utmost delicacy ; and the glass bundle interferes so much 

 with the observations, that it seems generally best to discard it, 

 and employ simply the most delicate polariscope attainable. I 

 have constructed a polarimeter out of the materials I had at 

 hand : the analyzer is a small Nicol ; the test-object a biquartz, 

 formed of two pieces of right- and left-handed quartz which 

 overlap in the middle thus — 



and the glass bundle consists of ten thin plates. If the values 

 of the inclinations of the bundle were known, the instrument 

 would be admirably adapted for the measure of maximum polari- 

 zations, the overlapping portion of the biquartz forming a test- 

 object of amply sufficient delicacy. But with this instrument, 

 observing certainly for the most part under very unfavourable 

 circumstances, I have never yet been able to distinguish the 

 neutral points or negative polarization in the sky. 



Brewster's test-object appears to have been much more sensi- 

 tive. The polariscope was presented to him by Babinet. To 

 give the appearance shown in the drawing of the neutral line at 

 p. 213 (Edinb. Trans, vol. xxiii.), it must have been of great 

 delicacy. It consisted of the well-known arrangement of two 

 wedges of rock-crystal with the axes crossed. 



If any progress is to be made in the subject, it is to be de- 

 sired that both observations and experiments should be insti- 

 tuted. The Kadcliffe observer (Mr. Main) has offered to assist 

 in bringing the subject before astronomers. And we might 

 learn much if Professor Tyndall were to resume, or some other 

 physicist to take up, the experimental inquiry. 



LX. On Unitation. — V. Some of the Applications and Develop- 

 ments of the General Formula. By W. H. Walenn, Mem. 

 Phys. Soc. 



[Continued from p. 1 22.] 



11. r j^HE most general statement of the principal formula of 

 J- unitation is, that if a given number be expressed in 

 terms of its digits and of the powers of 10, the formula 



