ZOOLOGY AND BOTANY, MICROSCOPY, ETC. G79 



cement, the emery will remain enclosed in it and the wheel will 

 only do its full amount of work by the exertion of undue pressure. 

 A careful examination of the dust especially with regard to the 

 proportions of cement, emery, and iron or steel particles which it 

 contains, will show without doubt whether the wheel is well made 

 and is doing its work efficiently. Under the best conditions the 

 grindiugs should consist mainly of iron or steel with few particles of 

 emery and few spherules of molten metal ; if there is much emery 

 present, the wheel is wearing too rapidly ; while the presence 

 of much molten metal indicates that too much pressure is being 

 exerted. Fig. 126 represents the dust from a good wheel; here there 

 arc only a few angular particles of emery, while the particles of 

 metal are sharp and clean cut. Fig. 127 contains a large quantity of 

 emery and only little cement, while the particles of metal are as in 

 the previous case, and the wheel will wear out very quickly. Fig. 128 

 represents the dust of a wheel which contains too much cement. 

 The great pressure necessary to make it cut was sufficient to fuse 

 the particles of iron or steel. 



Attached to the Konigliche Technische Hoehschule at Charlotten- 



'burg, Berlin, is a department fur the preparation of microscopic 



sections where metals are cut, polished, etched, and mounted for the 



Microscope. With the sections are also to be obtained diagrams in 



one or more colours drawn to the scale of 50 : 1.* 



The Microscope in the Workshop.t — Prof. W. A. Eogers in a 

 paper read before the Boston Meeting of Mechanical Engineers, 

 refers as follows to the use of the Microscope in the workshop : — 



" In the ordinary operations of the workshop, the lathe and the 

 planer are the primary tools, while the caliper, with the graduated 

 scale, is the secondary tool. Let us take the most simple case. It 

 is required to turn down a piece of metal to a given diameter. In 

 order to make the assumed case as simple as possible, we will assume 

 the required diameter to be an even inch. The caliper is set for this 

 unit of length, either from a graduated scale or, more accurately, 

 from an end-measure inch with parallel faces. The setting in the 

 latter case is done by the sense of feeling. We thus introduce an 

 additional element of complexity, since sight is at once the primary 

 sense and the ultimate test of a given limit of extension upon which 

 the workman must rely. When the market is supplied with gradu- 

 ated scales from which any required length may be taken by the 

 sense of feeling, it will be in order to defend the practice of relying 

 upon this sense as a final test in measurements of extension. As a 

 differential test, it is both useful and accurate. As an absolute test 

 it had better be abandoned. It is a makeshift at best. Assuming 

 that the caliper has been set to an exact inch, the workman turns the 

 piece of metal to the required size by a series of approximations 

 with the ever-present risk of going beyond the required limit. 

 During the final part of the operation he stops the lathe to test the 



* Central-Ztg. 1'. Optik u. Mech., vii. (1886) p. 131. 

 t Cf. Engl. Mech., xlii. (188tJ) pp. 397-8. 



