April 24, 1685.] 



♦ KNOWLEDGE ♦ 



347 



used to determine the zero point of the division. Taking 

 for granted that we have this standard plate, the sphero- 

 lueter is placed upon it, and the readings of the divided 

 head and indicator U not<^d when the point of the screw a 

 just touches the surface/". Herein, however, lies the great 

 difficulty in using this instruuient — i.e., to know the exact 

 instant of contact of the point of screw a on the surface/'. 

 Many devices have been added to the spheromotor to make 

 it as sensitive as possible, such as the contact level, the 

 electric contact, and the compound lever contact. The latter 

 is probably the best, and is made essentially as in Fig. 5. I 

 am indebted for this plan to Ur. Alfred JIayer. As in the 

 previous figure, a is the screw ; thb screw is bored out and a 



1/ 

 d 



Fig. 5. 



central steel pin turned to fit resting on a shoulder at c. 

 The end d projects below the screw a and the end e projects 

 above the milled head and the knife edge or pivot point 

 reats against the lever /, which in turn rests against the 

 long lever g, the point h of which moves along the division 

 at/ It is evident that if the point of the pin just touches 

 the plate, no movement of the index lever </ will be seen ; 

 but, if any pressure be applied, the lever will move 

 through a multiplied arc, owing to the short fulcri of 

 the two levers. Notwithstanding all these precautions, 

 we must also take into account the flexure of the 

 material, the elasticity of the points of contact, and other 

 idiosyncrasies, and you can readily see that practice 

 alone in an instrument so delicate will bring about 

 the very best results. Dr. Alfred Mayer's method of 

 getting over the great dithculty of knowing when all four 

 points are in contact is quite simple. The standard plate 

 is set on the box g, Fig. 1, which acts as a resonator. The 

 screw a is brought down until it touches the plate. When 

 the pressure of the screw is enough to lift off either or all 

 of the legs, and the plate is gently tapped with the finger, 

 a rattle is heard, which is the tell-tale of imperfect contact 

 of all the points. The screw is now reversed gently 

 and slowly until the moment the rattle ceases, and 

 then the reading is taken. Here the sense of hear- 

 ing is brought into play. This is also the case when 



the electric contact is used. This ia so arranged 

 that the instant of touching of the point of screw a, 

 completes the electric circuit in which an electro- 

 magnet of chort thick wire is placed. At the moment of 

 contact, or |)Prhaps a little before contact, the bt-ll rings and 

 the turning of the screw must be instantly stopped. Hero 

 are several elements that must be remembered, i'^irst, it 

 takes time to set the bell ringing, time for tlii^ sound to 

 pass to the oar, time for the sensation to be carried to the 

 brain, time for the braiu to send word to the hand to 

 cease turning the screw, and if you please, it takes 

 time for the haiid to stop. You may say, of what use 

 are such refinements f I may reply, what use is there in 

 trying to do anything the very best it can be done. If our 

 investigation of Nature's ])rofound mysteries can be par- 

 tially solved with good instrumental means, what is the 

 result if we have better ones placed in our hands ; and 

 what, we ask, if the best are given to the physicist 1 We 

 have only to compare the telescope of Galileo, the prism 

 of Newton, the pile of Volta, and what was done with 

 them, to the marvellous work of the telescope, the spectro- 

 scope, and dynamo of today. But I must proceed. It 

 will be recoguised that in working with the spherometer 

 only the points in actual contact can be measured at one 



time. 



that the four points 



Fig. G. 



a a a a may all be normal to a true plane, and yet 

 errors of depression as at e, or elevation as at b, exist 

 between them, so that the instrument must be used over 

 every avaUable part of the surface if it is to be tested 

 rigorously. As to how exact this method is I cannot say 

 from actual experience, as in my work I have had recourse 

 to other methods that I shall describe. I have already 

 quoted you the words of Professor Harkuess. Dr. Hastings, 

 whose practical as well as theoretical knowledge is of the 

 most critical character, tells me that he considers it quite 

 easy to measure to „uinu °^ ^"^ ^'^'^^ with the ordinary 

 form of instrument. Here is a very fine spherometer 

 that Dr. Hastings works with from time to time, and 

 which he calls his standard spherometer. It is delicately 

 made, its screw being 50 to the inch, or more 

 exactly 001998 inch, or within 2100,000 of being 

 1-50 of an inch pitch. The principal screw has a point 

 which is itself an independent screw that was put in 

 to investigate the errors of the main screw, but it was 

 found that the error of this screw was not as much as the 

 •00001 of an inch. The head is divided into two hundred 

 parts, and by estimation can be read to 1-100,000 of an 

 inch. Its constants are known, and it may be understood 

 that it would not do to handle it very roughly. I could 

 dwell here longer on this fascinating subject, but must 

 haste. I may add that, if this spherometer is placed on a 

 I)late of glass and exact contact obtained, and then removed 

 and the hand held over the plate without touching it, the 

 difference in the temperature of the glass and that of the 

 hand would be sufficient to distort the surface enough to 

 be readily recognised by tlie spherometer when replaced. 

 Any one desiring to investigate this subject further will 

 find it fully discussed in that splendid series of papers by 

 Dr. Alfred Mayer, on the minute measurements of modern 

 science, published in the Srimtljic American supplements, to 



