520 



MICROSCOPY. 



of 45, by which the field of view was trans- 

 ferred from the bottom to the side of the tank. 

 Sir David Brewster had already suggested the 

 employment of an objective corrected for use 

 in water instead of air ; but Dr. Goring, while 

 considering this plan more scientific than his 

 own, feared that it would be difficult to make 

 the object-glass water-tight, and that its being 

 able to serve only in water would be a bar to 

 its usefulness. Raspail substituted for experi- 

 ments with heated bodies a glass tube closed 

 at the bottom, thus avoiding the danger of heat 

 loosening the cemented cover. Mr. E. Rich- 

 ards likewise arranged a tank microscope, for 

 the examination of organisms which could not 

 be removed from the bottom of the tank with- 

 out destroying them. The objective was car- 

 ried below the stage by means of a long adapt- 

 er, and covered with a long shield-tube, allow- 

 ing it to be brought within focusing distance 

 of the bottom of a tank eight inches deep, 

 with powers as high as f-inch (" Month. Mic. 

 Jour.," 1874, p. 88). In the early days of im- 

 mersion lenses, Tolles made some, of low as 

 well as high power, especially corrected to use 

 unprotected in the fluid (water, mucus, serum, 

 etc.) containing the objects. 



<} During the past few years 

 ~j* the protector shown in Fig. 

 / 39 has been made by T. H. 

 McAllister, of New York, 

 at the suggestion of Mr. 

 R. E. Dudgeon, of London 

 ("Nature," 1878, p. 196; 

 Behrens, Am. ed., p. 31). 

 It is a brass tube, attached 

 to the objective only, closed 

 at the bottom with a cover- 

 glass, and is suitable for use 

 in the study or dissection 

 of objects contained in watch-glasses or small 

 troughs. During the present year, Mr. J. W. 

 Stephenson, of London, has revived, probably 

 unconsciously, in his aquarium microscope, the 

 exact contrivance figured by Dr. Goring, only 

 attaching the body to a bar across the aquarium 

 instead of to the microscope-stand. 



A protector of thin plate-glass, screwed into 

 the back of the objective to shield the lenses 

 from dust, though employed years ago by the 

 late F. A. Nobert, has been recently patented 

 by H. R. Spencer & Co. Though useful in 

 certain cases of special exposure, its claim to 

 leave the performance of the objective wholly 

 unimpaired may well be questioned. 



The Beale neutral-tint camera lucida is still 

 extensively made and used, at least in America. 

 The well-known disadvantages of singly-reflect- 

 ing instruments are more than counterbalanced 

 by its simplicity and cheapness, in those cases 

 where choice depends largely on the latter 

 considerations. Aside from the substitution of 

 a colorless cover-glass for the original tinted 

 plate, its improvements consist in the conven- 

 ience and economy of the mounting adopted 

 by different makers, and in the many devices 



FIG. 39. OBJECTIVE 

 PROTECTOR. 



of cork, and wax, and wire, by which a cover- 

 glass can be easily mounted for his own use by 

 any person so inclined. Other singly-reflecting 

 cameras, which have been repeatedly proposed 

 under various forms and names and claims, 

 have failed of extensive adoption ; and it may 

 be considered as settled by experience that any 

 one requiring a better camera than the Beale 

 will need a doubly-reflecting form. 



Only lately has the original Wollaston cam- 

 era lucida been fairly superseded by other 

 doubly-reflecting forms, equal in optical per- 

 formance, and free from its defects of requiring 

 a horizontal position of microscope and con- 

 strained steadiness of eye. With one excep- 

 tion the recent improved styles permit, what 

 seems to be a radical improvement, a direct 

 vision of the microscopic field, the pencil and 

 paper being seen by reflection. Nachet's, which 

 may be considered the pioneer as well as the 

 type of these, at least in American usage, con- 

 sists of a long, flat prism, whose rectangular 

 end covers the eye-lens, permitting a direct 

 and easy view of the field, while rays from 

 the drawing-pencil enter obliquely from below 

 the other end, and are reflected, first through the 

 length of the prism, and then upward into the 

 main axis of vision. The second reflection, in 

 the latest form, is from a translucent film of 

 gold through which the rays of direct vision 



FIG. 40. GRTTNOW'S CAMERA LUCIDA. 



pass without great loss, as suggested by Prof. 

 G. Govi, lying in the direct line of vision be- 

 tween two obliquely-cut and balsam-cemented 

 sections of the prism. The film of gold tints, 

 in a slight degree, both the drawing and the 

 microscopic field. The excellent working quali- 

 ties of this apparatus are slightly impaired by 

 the nearness of the drawing-field to the foot of 

 the microscope, and by the nearly vertical po- 

 sition required, unless the drawing-board be 

 inclined, in order to prevent distortion by 

 securing coincidence between the plane of 

 the image and that of the drawing surface. 

 Seibert & Krafft's camera differs only in 

 substituting for the prism a pair of plane 



