EXPLANATION OF PLATE III. 



Fig. 20, 20 a , 21 and 22. Sectional views of the Abbe Illuminator of 1.20 N. A. 

 [\\ 18, 54), showing various methods of illumination (jj{| 54-59). Fig. 20, axial 

 light with parallel rays. Fig. 20 a , oblique light. Fig. 21, axial light with con- 

 verging beam. Fig. 22, dark-ground illumination with a central stop diaphragm. 



Axis. The optic axis of the illuminator and of the microscope. The illumina" 

 tor is centered, that is its optic axis is a prolongation of the optic axis of the mi- 

 croscope. 



5". Axis. Secondary axis. In oblique light the central ray passes along a sec- 

 ondary axis of the illuminator, and is therefore oblique to the principal axis. 



A. Fig. 21 represents the upper part of the illuminator. 



D D. Diaphragms. These are placed in sectional and in face views. The dia- 

 phragm is placed between the mirror and the illuminator. In Fig. 20 the opening 

 is excentric for oblique light, and in Fig. 22 the opening is a narrow band, the cen- 

 tral part being stopped out, and thus giving rise to dark-ground illumination 



a 60). 



Obj. Obj. The front of the objective. 



Fig. 23. Showing the method of mounting letters in stairs to show the order of 



coming into focus. 



a, b, c, d. The various letters indicated by the oblique row of black marks in the 

 sectional view. 



Slide. The glass slide on which the letters are mounted. 



Fig. 24. Glass rod showing the appearance in air (a), and in 50 per cent, glycerin 

 (b), (J 80). 



Fig. 25. Glass rod coated with collodion to show Double Contour. At the left 

 the collodion is represented as collecting in a drop {%> 91). 



Fig. 26. Mammalian blood-corpuscles on edge to show a surface view (a), and an 

 optical section (b). 



Fig. 27. Wollaston's Camera Lucida, showing the rays from the microscope and 

 from the drawing surface, and the position of the pupil of the eye. 



Axis, Axis. Axial rays from the microscope and from the drawing surface. 

 (3 121). 



Camera Lucida. A section of the quadrangular prism showing the course of the 

 rays in the prism from the microscope to the eye. As the rays are twice reflected, 

 they ha\e the same relation on entering the e>e that they would have by looking 

 directly into the ocular. 



A B. The lateral rays from the microscope and their projection on the drawing 

 surface. 



C D. Rays from the drawing surface to the eye. 



A D, A' D / . Overlapping portion of the two fields, where both the microscopic 

 image and the drawing surface, pencil, etc., mav both be seen. It is represented 

 by the shaded part in the overlapping circles at the right. 



Ocular. The ocular of the microscope. 



P. The drawing pencil. Its point is shown in the overlapping fields. 



Fig. 28. Figure showing the position of the microscope, the camera lucida, and 

 the eve, and the different sizes of the image depending upon the distance at which 

 it is projected from the eye. (a) The size at 25 cm. ; (b) at 35 cm. [\ 104). 



Fig. 29. Figure showing the appearance of the lines of the stage micrometer 

 (the coarse lines) ar.d of the ocular micrometer when using a high objective ($ 117). 



A. One method of measuring the spaces by putting the ocular micrometer line 

 opposite the center of the stage micrometer line. 



B. Method of measuring the space of the stage micrometer by putting one line 

 of the ocular micrometer at the inside and one at the outside of the lines of the 

 stage micrometer {\ 117). 



