SOME SPECIAL PKOCESSES 263 



Another instrument of special design is that suggested by 

 Mr. J. E. Stead, F.B.S., and made by Messrs. J. Swift & Son 

 (Fig. 73). This one differs again, as it is only available for use 

 in a vertical direction ; but it is made very stiff, so that in cases 

 where a photograph of a fractured surface of a large forging, 

 or similar large mass of metal, has to be obtained in situ, the 

 microscope may be clamped direct on to the specimen. It 

 is of massive construction, and has a solid stage made to swing 

 out of position, so that the objective may be brought into focus 

 on any large object upon which the foot is supported. To effect 

 this an inside-tube carrying an object-glass slides within the 

 outer barrel, and can be lowered to a sufficient distance. For 

 photo-micrographic work a small conical camera is placed on 

 top of this barrel, and a photograph may then be taken directly. 

 This instrument is hardly intended for use with high powers or 

 for work of a critical nature ; but it fulfils the purpose for which 

 it was designed with very great efficiency, and it enables objects 

 to be dealt with that under any ordinary circumstances could 

 not be examined with a microscope at all. 



It has been necessary to give a detailed account of these 

 instruments, as they differ in important respects from the 

 ordinary microscope, as already pointed out ; and for this reason, 

 therefore, they were not dealt with in the chapter devoted to 

 microscope design and construction. As to the objectives for 

 metal-work, they differ only in one particular, and that is 

 that except in the case of oil-immersion lenses they must be 

 corrected to work correctly on objects that are not mounted 

 beneath a cover-glass. Metals usually have polished or etched 

 surfaces, and when vertical illuminators are used it is not 

 admissible to interpose a cover-glass between the objective and 

 the object. It is, therefore, absolutely necessary to ensure 

 that all dry lenses are specially corrected, and it is not possible 

 to use them at their best, except with very low powers, unless 

 this is the case. 



Illumination must be effected on the same general principles 

 as in work by transmitted light ; that is, with low-power work 

 a beam must be projected on to the specimen so that the field 

 is evenly illuminated, and in high-power work a critical image 

 of the illuminant must be exactly projected on the surface 

 of the object. This is in general a matter of no difficulty. 



