IO 



MICROSCOPE AND ACCESSORIES. 



[CH. I. 



MECHANICAL PARTS. 



§ II. The Mechanical Parts of a laboratory, compound microscope are shown in 

 the frontispiece, and are described in the explanation of that figure. The student 

 should study the figure with a microscope before him and become thoroughly 

 familiar with the names of all the parts. See also the cuts of microscopes at the 

 end of Ch. II. 



OPTICAL PARTS. 



f! 12. Microscopic Objective. — This 

 consists of a converging lens or of one 

 or more converging lens-systems, which 

 give an enlarged, inverted, real image of 

 the object (Figs. 14, 21). And as forthe 

 formation of real images in all cases, 

 the object must be placed outside the 

 principal focus, instead of within it, as 

 for the simple microscope. (See \\ 9, 

 49, Figs. 16, 21). 



Modern microscopic objectives usu- 

 ally consist of two or more systems or 

 combinations of lenses, the one next 

 the object being called the front com- 

 bination or lens, the one farthest from 

 the object and nearest the ocular, the 

 back combination or system. There may 

 be also one or more intermediate sys- 

 tems. Each combination is, in general, 

 composed of a convex and a concave 

 lens. The combined action of the sys- 

 tems serves to produce an image free 

 from color and from spherical distor- 

 tion. In the ordinary achromatic ob- 

 jectives the convex lenses are of crown 

 and the concave lenses of flint glass 

 (Figs. 22, 23). 



Fig. 21. Diagram showing the prin- 

 ciple of a compound microscope with 

 the course of the rays from the object 

 (A B) through the objective to the real 

 image (B' A'), thence through the ocu- 

 lar and into the eye to the retinal image 

 {A 2 B' 1 ), and the projection of the retinal 

 image into the field of vision as the 

 virtual image (B 3 A 3 ). 



AB. The object. A-B\ The retinal 

 image of the in verted real image, (i? 1 W), 

 formed by the objective. B 3 A 3 . The in- 

 verted virtual image, a projection of 

 the retinal image. 



