CH. II] LABORATORY MICROSCOPES 67 



66 



Figs. 64, 65, 66. Specimens Showing the Use of the Marker. 



In Fig. 64 a section of a series ts marked to indicate that this section shows some- 

 thing especially well. In Fig. 65 some blood corpuscles showing ingested carbon 

 very satisfactorily are surrounded by a minute ring, and in Fig. 66 the lines of a 

 micrometer are ringed to facilitate finding the lines. 



\ 126. Pointer in the Ocular. — The Germans have a pointer ocular (Spitzen- 

 Okular), an ocular with one or two delicate rods or pointers at the level of the 

 real image, that is, at the level of the diaphragm (Figs. 21, 30 D). For the pur- 

 poses of demonstrating any particular structure or object in the field, a temporary 

 pointer may be easily inserted in any ocular as follows : Remove the eye-lens and 

 with a little mucilage or Canada Balsam fasten an eye-lash (cilium) to the diaphragm 

 (Fig. 30 D) so that it will project about half way across the opening. If one uses 

 this ocular, the pointer will appear in the field and one can place the specimen so 

 that the pointer indicates it exactly, as in using a pointer on a diagram or on the 

 black-board. It is not known to the author who devised this method. It is cer- 

 tainly of the greatest advantage in demonstrating objects like amoebas or white 

 blood corpuscles to persons not familiar with them, as the field is liable to have in 

 it many other objects which are more easily seen. 



I 127. Mechanical Stage. — For High School and ordinary laboratory work a 

 mechanical stage is not needed ; but for much work, especially where high objec- 

 tives are used a mechanical stage is of great advantage. It is also advantageous if 

 the mechanical stage can be easily removed (see Figs. 67 to 70). The one found 

 on the most expensive American and English microscopes for the last twenty years 

 and the one now present on the larger continental microscopes, is excellent for 

 high powers and preparations of moderate dimensions, but for the study of serial 

 sections and large sections or preparations in general, mechanical stages like those 

 shown in Figs. 68-69 are more useful. This form of mechanical stage has the 

 advantage of giving great lateral and forward and backward motion. It is a mod- 

 ification of the mechanical stage of Tolles. The modification consists in doing 

 away with the thin plate and having a clamp to catch the ends of the glass slide. 

 The slide is then moved on the face of the stage proper. This modification was 

 first made by Mayall. It has since been modified by Reichert, Zeiss, Leitz, and 

 others in Europe and by the Bausch & Lomb Optical Co., Queen & Co., and the 

 Spencer L,ens Co., in America. — Jour. Roy. Micr. Soc, 1885, p. 122. See also 

 Zeit. Wiss. Mikroskopie (II), 1885, pp. 289-295 ; 1887 (IV, pp. 25-30). 



Those figured below have the great advantage of ready removal from the stage 

 of the microscope, thus leaving it free. They have also the very excellent feature 

 that with them one can explore an entire slide full of serial sections, as the sec- 

 tions are ordinarily mounted, i. e., under a cover-glass 24X50 mm. 



