F. C. THOMPSON 163 



light." The lenses in the objective are constructed of fused silica, 

 those of the eye-piece of quartz. A mixture of glycerine and water 

 of the required refractive index is used as an immersion liquid, and 

 focussing is carried out by the use of a fluorescent screen. So far 

 as the author is aware the highest power silica objective so far made 

 is the 1.7 mm. with 1.25 JST. A. In virtue, however, of the shortness 

 of the wave-length used, 275 jufi, this is equivalent to about 2.5 N.A., 

 using white light, and according to the rule given above could be used 

 for critical work up to 2,500 diams., which would be obtained with, 

 say, a x 15 eye-piece and a camera length of 31 c.m.s. There appears 

 to be a very big field of usefulness for this system in metallographic 

 research work. 



II. — The Davidson " Super-Microscope." 



The claims made for this instrument in connection with very 

 high magnifications — up to 15,000 diams. — render a consideration of 

 its capabilities germane to this paper. In essence, an image is 

 formed in the usual manner by a microscope objective, which image 

 is then magnified by a second microscope, with or without an eye- 

 piece. Such a procedure demands for the highest magnifications 

 an amplification of the primary image so high that no objective at 

 present made will stand it. In theory the instrument is unsound, 

 and in practice, so far as the author's observations go, the results 

 at high magnifications are very poor. Through the kindness of Sir 

 Robert Hadfield the apparatus was sent to the author some two or 

 three years ago for investigation. Since then the mechanical side 

 has been considerably improved, the optical arrangement remaining, 

 however, unchanged. Quite recently another opportunity has arisen 

 for his observation of the instrument, but his view is unchanged. 

 In the trial two test objects were chosen, one a very fine eorbitic 

 structure in a quenched and tempered tool steel, the other a normalised 

 Bessemer steel with 0.4 per cent, carbon, and 1 per cent, manganese, 

 the pearlite in which was finely laminated. With a magnification 

 of 1,000 diams., using Zeiss lenses and the ordinary apparatus, both 

 objects were easily resolved. At the same magnification, however, 

 the "Super-microscope" failed entirely. The sorbitic material 

 appeared structureless, while in the case of the pearlite only the 

 faintest suggestion of lamination could be detected. At higher 

 magnifications the apparatus is utterly valueless. 



The very claim put forward for the instrument that it possesses 

 great depth of focus is in itself an admission that the resolving power 

 is poor, since these two factors vary inversely as each other. Since 

 for the purpose at present under consideration, viz., high magnification 

 of metalhc structure, depth of focus is of no importance, while 

 resolution is a matter of prime weight, the remarks previously made 

 are still further substantiated. 



For other purposes, as a telescope, or for low power examination 

 of a metallic fracture where great depth of focus is quite necessary, 

 the apparatus probably offers very real advantages. In particular, 

 perhaps, may be mentioned direct measurements from some distance 

 of changes in length, such as are needed in a determination of 

 co-efficient of thermal expansion. 



