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succeeded by the application of another slight change in the inter- 
ferometer in designing a combination of the interferometers, in which 
the quantities / and /' always differ a same amount, which can be 
accurately measured. We shall further point out the way to determine 
this difference, and then demonstrate that the fact that this difference is 
constant enables us to work just as easily with this instrument and 
to reach just as much with it as would be the case with the ideal 
combination of the interferometers. 
From the modified interferometer we should, namely, immediately 
get back the original one, when the front planes of the prisms were 
silvered. Accordingly the two instruments can be joined to one by 
leaving the upper halves of the front planes of the prisms trans- 
parent, and by silvering the lower halves. Let the line of division 
between the two halves lie in the plane of the drawing, then the 
rays that run above this plane, and have, therefore, been drawn in 
full, are totally reflected and are joined in a point Q’, when they 
struck the prisms at right angles. Rays running under the plane of 
the drawing, and which have therefore been indicated dotted, will 
be reflected by the silvered lower halves, and collected by another 
eyepiece K into another point Q, when they too strike the prisms 
at right angles. Strictly speaking we have here placed the two inter- 
ferometers on top of each other, which was possible owing to the 
property of the cylinder lenses of never bringing a ray that runs 
above the plane of the drawing and parallel to it, under it. Rays 
that intersect the plane of the drawing could, indeed, pass from one 
instrument into the other, but these are not joined in the points Q 
or Q’, and it will appear, that we have to measure the intensity 
only in these points. Moreover the front plane of every prism can 
be divided into three regions, the top one transparent, the middle 
one absorbing, and the bottom one silvered, through which, as it 
were, a Space arises between the two interferometers, depending on 
the height of the middle region. The upmost and the downmost 
regions must remain large, as all the incident light that strikes at 
right angles is concentrated in the points Q and Q’, and we want 
to have a strongly pronounced gradient of intensity in these points, 
when presently the movable prism is shifted. 
We saw already that the intensity in the point Q’ was determined 
by equations (12) and (13). The intensity in the point Q is repre- 
sented by equation (4), in which / was the distance from the movable 
mirror to the plane of reference, and for which, therefore, V7 of 
fig. 7 must be taken. We shall, therefore, call the length of VZ'/. 
Summarising we find in Q’ and Q resp. the intensities - 
