795 
axis of collimator and eye-piece. That (1) does not represent the 
intensities accurately follows among others already from the fact 
À 
that the distance of two orders does not amount toa, =~, i. 6. is 
9 
not independent of the position of the echelon. This distance changes 
very appreciably on rotation of the echelon; it increases as the 
echelon moves further from the position, at which the light is 
parallel to the steps. The difference between the greatest and the 
smallest distance of the orders amounted to abont 10°/, of this 
distance. We have, however, not occupied ourselves more closely 
with these particularities, but confined ourselves to expressing all the 
distances as fraction of the distance of the orders in the spectrum 
in question. 
The measurements have been carried out with regard to three 
components of the green mercury line, namely on the so-called 
principal line and two satellites (84 = — 0,0242uu and 8A=—0,0078uu, 
Nac. and Tak. loc.cit.). In its different orders and with tbe different 
positions of the echelon each of these lines gives a series of points 
of a curve which indicates the relation of intensity and position. The 
three curves obtained in this way have been reduced to one and 
the same value of the maximum intensity, which is reached when 
the line is in the centre of the image plane.') Fig. 3 gives the 
observed point, in which . refers to the principal line, O and X 
resp. to the stronger and the weaker satellite. The uninterrupted 
curve represents the theoretical distribution of intensity. 
Fig. 3. 
The agreement is sufficient in the neighbourhood of the maximum. 
1) The observations show that really every line has its maximum at the same 
point of the image plane. . 
