DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
771 
Modes of Testing Number of Light Journeys. 
42 . There are many ways of making sure how many times round the frame the light 
is going. One is to look into the telescope with a low-power eye-piece, or none a.t all, 
before the semi-transparent plate is inserted and to give a wide opening to the 
collimator, often also removing its lens. Three or more images can then be seen in 
different parts of the large field, and it is easy to see v/hich is the one near the centre. 
On now tipping the front mirror to and fro, the image which has been only once 
round (if visible) remains unaffected; the one which has been twice round moves; 
the three-times-round one moves twice as much ; and so on. 
By altering adjustments and passing the successive images in review, it is not 
difficult thus to work up to the high numbers. But on the specification of these it is 
well to have some check. The best check can be got, with everything in position and 
bands visible, by passing an opaque strip slowly in front of one of the mirrors and 
observing the eclipses at the telescope. If the semi-transparent plate is inserted in 
its holder, these eclipses occur in pairs, with shadows moving oppositely ; if not, they 
occur singly. (A very narrow strip passed not quite close to the mirror may show 
each single shadow double. Too broad a strip will, of course, merge a pair of shadows 
into one.) 
Three times round naturally gives two pair of eclipses on the front mirror, and 
three pair on each of the others. 
Without the cover-glasses the light can be got many times round, but, when they 
are on, the faintness of the light which gets through all the surfaces makes it unwise 
to aim at more excursions, because the definition and visibility of the bands suffers in 
undue proportion. In fact, the superior sharpness of the twice-round bands perhaps 
more than compensates for the advantage of the half-as-long-again path belonging to 
the three-times-round set. 
The cover-glasses are, of course, not set absolutely vertical, else the enormous 
number of reflexions from their surfaces would confuse everything. 
Another way of testing the order of the image in use is to tilt the front mirror so 
as to broaden the bands by a measured amount, and then to imitate or compensate 
this by tilting one of the other mirrors. The angle of tilt necessary with one of these 
will be n/(w -j- 1) of the tilt of the front one. 
The three screws against which they each press all have fifty turns to the inch, and 
hence it is possible to apply this test whenever the eclipse method happens to be 
inconvenient. For instance, i^fh of a revolution of the single screw of the front 
mirror broadened the bands till one occupied nearly the whole field (with the high- 
power eye-piece that happened to be on). This meant s^th of an inch advance of 
one end of a plane of 10 Inches base, or an angular tilt of 50^0 0 radian, or 40 seconds 
of arc. Tilting the back mirror about 20^^ gave the same effect, showing that there 
were twice as many reflexions on it as on the front one, and that the light was 
therefore going twice round. 
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