1886.] on Telescopic Objectives and Mirrors. 419 



To give an idea of the delicacy of the instrument, I bring the 

 screw firstly into contact with the glass. Now the screw is in good 

 contact ; but there is so much weight still on the three feet, that if 

 I attempt to turn it round, the friction on the feet oppose me, and it 

 will not stir except I apply such force as will cause the whole instru- 

 ment to slide bodily on the glass. Now, however, I raise the whole 

 instrument, taking care that my hands touch none of the metal-work, 

 and that the screw be not disturbed. I lay my hands for a moment on 

 part of the glass where centre screw stood, and thus raise its tempera- 

 ture slightly, and on laying the spherometer back in the same place, you 

 now see that it spins on the centre screw, showing how easily it 

 detects what to it is a large lump, caused by expansion of the glass from 

 the momentary contact of my hand. 



Flexure. 



One of the greatest difficulties to be contended with in the 

 polishing of large lenses is that of flexure during the process. 



It may appear strange that in discs of glass of such considerable 

 thickness as are used for objectives, any such difficulty should occur ; 

 but a simple experiment will demonstrate the ease with which such 

 pieces of glass can be bent, even under such slight strain as their own 

 weight. 



We again take our spherometer and set it upon a polished 

 surface of a disc of glass of about 7 J inches diameter and J inch 

 thick. I set the micrometer head as in the former experiment to bear 

 on the glass, but not sufficiently tight to allow the instrument to spin 

 round. This has now been done while the glass as you see is sup- 

 ported on three blocks near its periphery. I now place one block 

 under the centre of disc and remove the others thus, and you see the 

 instrument now spins round on centre screw. 



It is thus evident that not only is this strong plate of glass 

 bending under its own weight, but it is bending a quantity easily 

 measurable by this instrument, which as I shall presently show is quite 

 too coarse to measure such quantities as we have to deal with in 

 figuring objectives. 



After this experiment, no surprise will be felt when I say that it 

 is necessary to take very special precautions in the supporting of discs 

 during the process of polishing, to prevent danger of flexure ; of 

 course, if the discs are polished while in a state of flexure the 

 resulting surface will not be true when the cause of flexure is 

 removed. 



For small-sized lenses no very special precautions are necessary, 

 but for all sizes over 4 inches in diameter I use the equilibrated 

 levers devised by my father, and utilised for the first time on a large 

 scale in supporting the 6-foot mirror of Lord Eosse's telescope. 

 These have been elsewhere frequently described, but I have a small 

 set here as an example. 



I have also sometimes polished lenses while floating on mercury. 



