108 
MR. J. G. LEATHEM OX THE THEORY OF THE 
When the incident light is polarised perpendicularly to the plane of incidence, 
Bq = 0, and the incident ray is represented by —Aq/cosz ; the magneto-optic 
component of the reflected ray by — B, that is to say that term in — B which 
contains the factor Aq. The ray relatively to which phase is measured is represented 
by that term in — B which contains the (vanishing) factor B^. If Bq be supposed 
to be only just not zero, then, since the incident ray is supposed to be plane 
polarised, B.j/A^ is a real quantity. Hence we have 
1 r (w'/ib/i) Coe‘"'(/«o + H70) 
.9^ = vector angle ol -^- - 
• • (41). 
Fi’om (40) and (41) we see at once that when the reflection is equatorial, that is, 
when = 0, 
= .9^, = .9 (say), 
and this agrees with Sjssingh’s observations. 
We also see that when the reflection is polar, that is when 
db 180°. 
= 0 . 
Now Zeeman, as a result of experiments on polar reflection described by him in the 
‘ Archives Neerlandaises,’ vol. 27, came to the conclusion that iiii = It is very 
possible that this discrepancy is due to his using a definition of 7??- and slightly 
different from Sissingh’s. 
22. When the reflection is equatorial, we see from (40) that 
.9 = vector angle of 
— 2cX sin i Cgao COS i 
(iH - cos i) (iiHR-- + cos i) ‘ 
In determining .9 from this expression there is an ambiguity to the extent of 180°; 
for in defining m (or 360° — SissiNGH requires that it shall not be altered when 
aQ changes sign. Examining his paper, we see that in equatorial reflection the 
standard case is when is negative. Hence, remembering that Cg is negative, we 
find that 
.9 = a; — 90° — 2a — the sum of the vector angles of 
iW, (#t — cos i), and (^R“^ e~~‘“ + cos i) . . (42); 
and to get ^ accurately for any particular angle of incidence, these three vector 
angles must be calculated. 
The following table shews the results of SissiNGH’s observations on the phase for 
