26 VEGETABLE HISTOLOGY. 



all points of the image are in focus at the same time. This 

 latter quality is essential in all good microscopes. In all nega- 

 tive eye-pieces the image of the object is formed between the 

 two lenses, and is then further magnified by the eye lens. The 

 lenses taking part in the formation of the first image are, there- 

 fore, the objective and the field lens of the eye-piece. In posi- 

 tive eye-pieces the first image is formed below the field lens, 

 i. e., the ocular takes no part in its formation. An example of 

 such is- Ramsden's eye-piece. 



Eye-pieces are designated by methods like those stated for 

 objectives. Those most commonly used have focal lengths of 

 2 inches, iy 2 inches and 1 inch. 



There is a particular order that should be observed in chang- 

 ing the lenses in passing from a low power to a high one. For 

 example, there are two objectives, 2 / 3 and 1 / 8 inch, and two eye- 

 pieces, 2 and 1 inch. The following is the best order for chang- 

 ing these : 



Objective. Eye-piece. 

 2 / 3 inch 2 inch Low power. 



1 / 8 inch 2 inch Medium power. 

 */8 inch 1 inch High power. 



In other words, it is better to increase magnification at the 

 objective end than at the eye-piece end of the microscope. The 

 reason for this is that the eye-piece magnifies any defects of the 

 objective. In good instruments a high objective is not likely 

 to have more defects than a low one, hence in increasing mag- 

 nification by changing to a high objective the image will have 

 no more defects than before, although much more magnified. 



As the magnifying power is increased the field of view be- 

 comes smaller, illumination of the image decreases and the 

 image is increased in size. 



TUBE LENGTH. Magnification may also be increased by draw- 

 ing out the inner tube, which increases the distance between the 

 objective and the plane of the real image formed in the barrel 

 of the instrument, and, consequently, the size of the image. 

 The tube length, however, should be kept constant, because the 

 objectives are prepared to suit a definite tube length, which is 

 sometimes fixed at 160 mm. (6.3 inches), sometimes at 216 mm. 

 (81/2 inches). The scale on the inner tube regulates the length. 



CAMERA LUCIDA. This is a drawing apparatus which is at- 

 tached to the eye-piece, and is used whenever it is desired to 

 make accurate delineations of the object. By means of it, a 

 white surface of paper, on the table alongside of the instru- 

 ment, is reflected into the eye while it receives the image, and 

 thus a pencil point can be traced on the paper along the lines 

 of the image, giving an accurate drawing. The Abbe camera 



