QUARTZ FIBERS. 331 



rois are arranged to move by clock-work, so as to make the two images 

 travel at proportional rates. Tl»e moon is travelling with the dark edge 

 foremost, and now that the terminator of the moon has come npon the 

 sensitive surface, the heat is felt and the deflection of the instrument 

 is the result. Now, as the moon is gradually travelling through the 

 sky, the radiation is slowly and steadily increasing, because the radia- 

 tion from the moon gets greater and greater, as the point at which 

 the sun is shining vertically — that is, a point at right angles with the 

 terminator — is approached ; it is here a maximum, and then it falls 

 back, and as soon as the moon has gone off the instrument, you will 

 see the index fall back almost suddenly. But there is something more. 

 This moon in one respect is better than the other moon. At the pres- 

 ent time it represents the moon nineteen days old, a moon, that is to 

 say, which is waning, and which goes through the sky with its dark 

 edge foremost. The clock-work will now bring the moon back again, 

 and convert the nineteen-day moon into a nine-day moon, one in which 

 the bright edge goes forward. What I want you to notice (and it will 

 be perfectly evident) is this, that the spot of light will now go up the 

 scale suddenly, will then rise to a maximum position, and will then fall 

 slowly until the terminator is reached, which proves that in the former 

 case the slow rise and sudden fall, or the present sudden rise and slow 

 fall was not a peculiarity of the instrument, but was due to the fact 

 that the different points of the moon radiated in the manner which I 

 have stated. There is one point which, as the moon has now left the 

 instrument, I should like to show; that is, that it is a real moon and 

 not a mere slide. That is shown by gradually moving the sun round. 

 Now it is at right angles to the line of view, and we have got the half- 

 moon. As it goes round, the moon continues waning, appearing more 

 like a new moon, and at last we have an eclipse of the sun, which :nay be 

 annular if the i)roportions of the apparatus are properly arranged. 



I wish now to make a few statements as to the delicacy of apparatus 

 that can be made with the help of quartz fibers. I would wish you 

 most distinctly to understand that it is not sufficient to go into a shop 

 and buy apparatus as it is now made, replace the silk by quartz, and 

 to suppose you can get a degree of delicacy such as 1 have shown you. 

 That is not sufficient. If you take out the silk and put in a quartz fiber 

 the apparatus will be much improved, and you can then increase its 

 delicacy. You will then escape the troubles due to silk; but one after 

 the other a new series of disturbances will appear, and anything like 

 ultimate, extreme, and minute accuracy will still seem out of the ques- 

 tion. Now, it has been my business to eliminate one by one these dis- 

 turbing influences. I will not weary you with a description of them all, 

 and the methods by which they may be certainly provided against. 

 These disturbing causes, which at the present time with instruments 

 carrying a silk fiber are not even known to exist, or if known to exist, 

 are practically of no consequence whatever, come one by one into prom- 



