578 Professor Oliver Lodge [April 1, 



no doubtful properties of transparent substances, but on the straight- 

 forward fundamental principle underlying all such simple facts as 

 that — It takes longer to row a certain distance and back up and down 

 stream than it does to row the same distance in still water ; or that 

 it takes longer to run up and down a hill than to run the same 

 distance laid out flat ; or that it costs more to buy a certain number 

 of oranges at three a penny and an equal number at two a penny 

 than it does to buy the whole lot at five for twopence. 



Hence, although there may be some way of getting round Mr. 

 Michelson's experiment, there is no obvious way ; and I conjecture 

 that if the true conclusion be not that the ether near the earth is 

 stagnant it will lead to some other important and unknown fact. 



The balance of evidence at this stage seems to incline in the sense 

 that the earth carries the neighbouring ether with it. 



But now put the question another way. Can matter carry neigh- 

 bouring ether with it when it moves ? Abandon the earth altogether ; 

 its motion is very quick, but too uncontrollable, and it always gives 

 negative results. Take a lump of matter that you can deal with, and 

 see if it pulls any ether along. 



That is the experiment I set myself to perform, and which in the 

 course of the last year, I have performed. 



I take a steel disk, or rather a couple of steel disks clamped 

 together with a space between. I mount it on a vertical axis and 

 spin it like a teetotum as fast as it will stand without flying to pieces. 

 Then I take a parallel beam of light, split it into two by a semi- 

 transparent mirror (Michelson's method), a piece of glass silvered so 

 thinly that it lets half the light through and reflects the other half ; 

 and I send the two halves of this split beam round and round in 

 opposite directions in the space between the disks. They may thus 

 travel a distance of 20 or 30 or 40 feet. Ultimately they are allowed 

 to meet and enter a telescope. If they have gone quite identical 

 distances they need not interfere, but usually the distances will differ 

 by a hundred-thousandth of an inch or so, which is quite enough to 

 bring about interference. 



The mirrors which reflect the light round and round between the 

 disks are shown in Fig. 10. If they form an accurate square the last 

 two images will coincide, but if the mirrors are the least inclined to 

 one another at any unaliquot part of 360° the last image splits into 

 two, as in the kaleidoscope is well known, and the interference bands 

 may be regarded as resulting from those two sources. The central 

 white band bisects normally the distance between them, and their 

 amount of separation determines the width of the bands. There are 

 many interesting optical details here, but I shall not go into them. 



The thing to observe is whether the motion of the disks is able to 

 replace a bright band by a dark one, or vice versa. If it does, it 

 means that one of the half beams, viz. that which is travelling in the 

 same direction as the disks, is helped on a trifle, equivalent to a 

 shortening of journey by some quarter millionth of an inch or so in 



