216 
my, my 
Pl 2 fx (mm) dm —2 fx (m) sin Ax l'm dm 
ma ma 
my, my MP 0e (14) 
J()= 2 fu dm 42 fx (m) cos 47 lm dm 
ma ms 
in which ! = lv when v = ulq—p—d) +d 
A diaphragm with two fine apertures at the place of the points 
Q and Q’ casts two beams of light of the above mentioned intensity 
on a photographic plate. When the movable prism is slowly shifted, the 
intensities of these beams change on account of the change in the para- 
meters land /’ in (14). The difference /’—/ remains, however, 
constant, namely equal to v, which quantity is an instrument con- 
stant, as appears from (10), if u does not appear in it. For that the 
prisms must be made of different size and so, that g=p+t+d. A 
small error in their size once for all can be neutralised by turning 
the plate PL over a small angle. Now » becomes = d, hence an 
instrument constant. When the photographic plate is shifted over 
large distances and in its own plane with a slight movement ot 
the prism through coupling, and when care is taken that the shifting 
of the plate is always proportional to that of the prism, and further 
that the shifting of the prism always takes place with uniform 
velocity *), the two beams will leave behind two bands on the 
photographic plate, where the blackening is different point for point, 
and which, when they are afterwards measured photometrically 
with the photometer and thermopile of Dr. Morr, yield two curves, 
which will be of great importance for the determination of the 
desired function (a). 
We shall call the curves that arise when the bands are measured, 
Z(el) or Z'(cl’), according as their ordinates represent the course 
of the blackening in the lower or the upper band, which is, therefore, 
: ! if at 
made in Q or in @. When „ denotes the ratio of the shifting of the 
prism, and the corresponding displacement of the photographic plate, 
and when in the upper band that point is assumed as zero point 
N that was placed in Q when the prism passed through the position 
/=0, then light, the intensity of which corresponds to the position 
lof the prism, and is therefore indicated by equation (4), will have 
fallen on that band in a point whose abscissa amounts to c/ measured 
1) In order to keep the time of exposure the same for every position of the prism. 
