152 Transactions of the Society. 



piece end, is then the full natural size of the source, and is seen by 

 the eye magnified by the eye-piece only. By using a large source 

 as indicated above, the whole field can be uniformly illuminated, 

 and in the author's experience such lighting is preferable to any 

 other for all " vertical illumination " work on metals. If a small 

 source of light is used, such as an electric arc, this direct method 

 is not available, particularly as it would not be practicable to bring 

 an arc so near the Microscope. One means of attaining the desired 

 object is to utilise the light of the source, by means of a suitable 

 system of lenses, to form a small bright disk of the required size, 

 upon a translucent screen placed at the proper distance from the 

 Microscope, and then to use the image of this screen for critical 

 illumination. Another method is to throw a real image of the 

 source to a point close to the illuminator aperture and then to 

 interpose between the image and the illuminator a suitable negative 

 lens giving an enlarged virtual image of the source at the proper 

 distance from the illuminator ; with a proper choice of foci for the 

 various lenses, it is possible in this way to obtain a critical image 

 of the arc large enough for the image of one crater to fill the field 

 as seen by the eye or in the camera ; but such spreading of the light 

 necessarily entails loss of intensity, so that the use of critical 

 illumination for photo-micrography of metallic specimens still 

 entails some difficulties. For visual purposes, however, it not only 

 yields the best results, but also furnishes the simplest method of 

 illumination, no condensers whatever being required if a large 

 source be used. 



With objectives of focal lengths up to about 25 mm. (1 in.) the 

 use of the internal reflector or " vertical " illuminator furnishes 

 the most satisfactory results ; but from that focal length onwards 

 the distance between objective and specimen is large enough to 

 allow of the use of external reflectors. If normal illumination is 

 required it can be obtained by interposing a reflector between the 

 objective and the specimen. The advantage of doing this is that 

 all internal reflections are avoided, and a beam of wider angle can 

 be condensed on the specimen by suitable condensers than could be 

 obtained from these long-focus objectives. The external reflector 

 may take the form either of a thin glass slip covering the entire 

 field of view and inclined at about 45° to the optic axis, or it may 

 take the form of either a metallic reflector (Sorby), or a silvered 

 glass reflector covering about half the aperture of the objective. 

 So far as the author's experience goes, the thin glass reflector gives 

 the most uniform illumination, but its presence distinctly interferes 

 with the definition. In the present instrument any of these 

 reflectors may be carried on the lower end of a long slide which 

 fits into the grooves outside the illuminator openings of the body- 

 tube ; the lower end of this slide carries a wide ring completely 

 surrounding the objective, but out of contact with it, and upon 



