146 



NATURE 



[June 16, 1898 



like a marking gauge. With such an instrument, tubes 

 may be cut with facility and accuracy. In the same way 

 it is not easy to cut a circular hole in an ordinary glass 

 shade ; but if a glazier's diamond is used on a compass 

 within the shade, the piece will drop out at once. 



There are useful instructions on boring holes in glass. 

 Of course, the nature of the tool and of the process 

 depends upon the size of hole and thickness of glass. It 

 will be news to most people, however, to read, after the 

 process of drilling with a file is described : 



" It is not, however, necessary to use a file at all, for 

 the twist drills made by the Morse Drill Company are 

 quite hard enough in their natural state to bore glass. 

 The circumferential speed of the drill should not much 

 exceed ten feet per minute. In this way the author has 

 bored holes through glass an inch thick without any 

 trouble, except that of keeping the lubricant sufficiently 

 supplied." 



The writer has always believed that a pyramidal end 

 to a drill — that is, a drill of the old-fashioned flat pattern, 

 but with the two faces meeting at the point, not jomed 

 by a cross-edge — was the best form for drilling glass, 

 i.e. when a diamond drill is not available. Such a drill, 

 made dead hard and well lubricated, certainly drills holes 

 in thick glass with remarkable facility. The success of 

 the Morse twist drill, where such cross-edge is always 

 present, would seem to indicate that there is nothing 

 essential in the pyramid theory. The application of the 

 methods of the mechanical engineer to the work of glass 

 is carried a step further on p. 74, where the reader is told 

 to give up grinding glass to form in many cases where 

 this is the usual practice, but instead to chuck it in the 

 lathe and turn it with a steel tool ground to an edge of 

 80° and well lubricated. After this, any one who has not 

 worked glass in this way would almost expect to read — 

 the best way to start the Morse drill in glass boring is to 

 use a dead hard and sharp centre punch, and give it a 

 smart blow with a light hammer. 



An appendix to the first chapter should be found 

 useful, since the interest in experimenting with Rontgen 

 tubes shows no sign of decaying. Complete and detailed 

 instructions are given for making all the parts of these 

 tubes, for putting them together, for making a suitable 

 pump, and for completing by exhaustion and sealing. 



The second chapter is upon glass grinding and op- 

 tician's work. This is one which the great majority of 

 experimentalists will look upon as outside their practical 

 requirements. There is no doubt that the art of optical 

 grinding, as distinct from mere lapidary performances, 

 is one of the most fascinating for the very few who have 

 laid themselves out to practise it. It, however, is one 

 which cannot be embarked upon in five minutes. A good 

 deal of material and apparatus has to be collected before 

 a start can be made, and at the best the processes seem 

 slow and tedious ; they are, moreover, of a kind that 

 cannot be hurried. On the other hand, where the prac- 

 tical physicist finds himself in some outlandish place, it 

 may very well be worth his while to acquire the art of 

 grinding and polishing plane and curved surfaces, and 

 of attaining the skill, if he has the patience, of figuring 

 these with the precision that optical work demands. 

 For those within reach of the real or working optician — 

 quite a distinct type from the shoptician — it is barely worth 

 NO. 1494, VOL. 58] 



spending the time for the sake of the work to be done,, 

 though it may be for the sake of the pleasure that 

 succeeding in a difficult art will bring to the worker. 

 But this is luxury. 



On the other hand, occasions arise in experimental 

 work where it is important to be able to do on the spot 

 and at once some operation of a kind which, taken by 

 itself, the experimentalist would prefer to put in the hand 

 of the instrument maker, but which it may be imperative 

 to perform on the spot, even though the technical success 

 may be inferior to that of a second-rate professional. 



The whole series of operations required in making an. 

 achromatic object-glass of small size are described, not 

 because any one wanting such a glass would be well 

 advised to make one, but because such a description 

 includes all the ordinary routine of optical work, and a 

 beginner would find it a good training. After this the 

 construction of small lenses and of galvanometer mirrors 

 is described. The author tried making these mirrors of 

 fused quartz and of crystalline quartz, as well as of glass,, 

 and has concluded that for the most perfect thin mirrors 

 slices of the crystal are better than anything. In this 

 conclusion the writer of this notice agrees. 



The construction of large mirrors and object-glasses 

 for telescopes is dealt with ; but in the writer's opinion 

 this, while good enough, is somewhat out of place, for it 

 is not possible to devote enough space to the very 

 wonderful art of testing the surface at the centre of 

 curvature. The formula for the longitudinal aberration 

 of the parabola at this point is not given, nor is the 

 reader warned that the formula of Draper, which is so 

 constantly quoted for this, only gives half the correction. 



Sections 68 and 69 should be valuable to many. They 

 are both quotations from Brashear, whose optical master- 

 pieces are known of, if they have not been actually seen 

 by every experimentalist in the world. The first is on 

 the cleaning of dirty object-glasses, and the second on 

 the working of plane surfaces on rock salt. 



Some attention is given to the peculiar difficulties of 

 producing optically plane surfaces of any size. Lord 

 Rayleigh's beautiful method of testing the figure by 

 interference with a free surface of water just above it is 

 referred to rather than described. While interference 

 methods of testing are shortly described — and they have 

 the undoubted value that they indicate the magnitude and 

 position of any errors — it is, perhaps, unfortunate that 

 the very handy method of testing the goodness of a 

 plane surface by the use of a telescope and artificial star 

 is not properly described. 



The chapter on optical work is really full of valuable 

 information. The fact that some criticism has been 

 offered is perhaps owing to the fact that the subject is 

 one upon which no two people would have quite the same 

 views. The writer must, however, here express his dis- 

 appointment at not finding any indication of the value 

 of carborundum for these processes. He has never 

 lost an opportunity of trying to collect real experience 

 on this material, practically without success. His own 

 very limited experience is all in favour of the virtues 

 which the makers so forcibly set out. It seems impossible 

 in this country to learn anything about it directly. 



The third chapter is on all sorts of things that 

 the manipulator in materials ought to know. The 



