248 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



arranged for use with compound lenses, has been devised by employing 

 the binocular of Mr. Stephenson. This instrument is illustrated in 

 fig. 200. It is made by Swift and Son. The stage may be enlarged 

 as a dissecting table, with special rests for the arms. The objective 

 and binocular part of the body remain vertical and focus vertically 

 by a rack-and-pinion coarse adjustment, there being no fine adjust- 

 ment. The bodies above the binocular prisms are suitably inclined, 

 mirrors being placed inside them to reflect the image. This reflec- 

 tion also causes the erection of the image, which is valuable to the 

 majority engaged in insect dissection or the dissection of very 

 delicate and minute organisms or organs. 



Another type of dissecting microscope has been introduced (as 

 we have seen on pp. 102-4) by the firm of Zeiss ; it is known 

 as Greenough's Binocular Microscope, and possesses valuable 

 and interesting features, and has been prepared to facilitate the 

 examination, dissection, and -preparation of eggs, Iarva3, and 

 other solid objects by furnishing a true stereoscopic and erect 

 image. Hence it is most useful for zoologists, botanists, and 

 embryologists. To accomplish this purpose a combination of Porro 

 prisms with a compound microscope of the usual optical type has 

 been effected. We have said enough of this instrument in an 

 earlier page, and merely recall its adaptation to dissecting purposes 

 by the illustration furnished in fig. 201, and we would remark that it 

 is only when two such complete microscopes, each having its own 

 objective and eye-pieces, are simultaneously directed upon an object 

 that the truest stereoscopic images can be obtained. 



Only comparatively low powers can be used with this instrument, 

 but this is no defect, for with such powers alone would the work it 

 is intended to do be accomplished ; but two special eye-pieces of 

 different powers, corresponding to Huyghenian eye-pieces 2 and 4, 

 are prepared for this microscope ; they are known as orthomorphic. 

 The magnifications resulting from the combination of these eye- 

 pieces with the objective are respectively 25 and 40. 



We have now to consider the most primitive stands adopted for 

 simple microscopes. That in the form of a bull's-eye stand is the 

 least complex form possible. This instrument holds an intermediate 

 place between the hand magnifier and the complete microscope, 

 being, in fact, nothing more than a lens supported in such a manner 

 as to be capable of being readily fixed in a variety of positions 

 suitable for dissecting and for other manipulations. It consists in 

 its best form of a circular foot, wherein is screwed a short tubular 

 pillar (fig. 202), provided with a rack-and-pinion movement, and 

 carrying a jointed arm movable in many directions by ball-and- 

 socket and other joints, b, c, e, but capable of being clamped by 

 thumb-screws or milled heads, a, b, e ; one end of this arm carries a 

 joint, to which is attached a ring for holding the lenses. By 

 lengthening or shortening the pillar, by varying the angle which 

 the arm makes with its summit, and by using the various joints, 

 almost any position and elevation may be given to the lens that can 

 be required for the purposes to which it may be most usefully 

 applied, care being taken in all instances that the ring which carries 



