ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 855 



projection-lens so adjusted that the edge of the diaphragm is focused 

 as sharply as possible on the screen or ground glass of the photo- 

 graphic camera. This is accomplished by drawing out the projection 

 lens more or less according as the distance between the screen or 

 plate and the Microscope is reduced or increased. Finally, the image 

 of the object is sharply focused on the screen or ground glass by the 

 usual adjustments. The length of body for which the objective is 

 adjusted for observation with an eye-piece, must always be exactly 

 retained. 



The cap of the projection eye-piece forms a diaphragm by which 

 any false light from the body-tube is completely shut off. The size of 

 the aperture of this diaphragm corresponds with the highest aperture 

 of the apochromatics. When using either those of 0*6 or 0*3 it 

 may occasionally be desirable to decrease the available aperture of 

 the objective in order to obtain uniform sharpness of definition up to 

 the margin of the field. For this purpose each projection eye-piece is 

 supplied with two diaphragms of smaller apertures which fit in place 

 of the normal one. It must not be forgotten to remove these from 

 the eye-piece if the full aperture of the objective is to be effective. 



Projection by this method gives extremely sharp, uniformly illu- 

 minated, pictures of any desired degree of magnification. 



The projection eye-pieces are specially corrected for the apochro- 

 matics on the principle of the compensating series of eye-pieces, but 

 may nevertheless be advantageously employed with ordinary achro- 

 matic objectives of large aperture. They are constructed for both 

 Continental and English Microscopes, on somewhat different formula}, 

 according to the difference in tube-length. There are two numbers 

 for each series, giving an 



■d • _»£ a- if (2 and 4 for the 160 mm. bodv. 



Eye-piece magnification of } , c r ,? „"" . "•'• 



. . IS and 6 for the 250 mm. body. 



These figures indicate, as in the compensating eye-pieces, the ratio 



in which, by means of the eye-piece and the given length of body-tube 



for which it is adjusted, the focal length of the whole Microscope 



is less than that of the objective alone (in so far as the eye-piece is 



adjusted to great distance). 



For instance, the projection eye-piece 2 diminishes the focal 

 length of each objective by exactly one-half; an objective of 3 mm. 

 therefore will, with this eye-piece, project as large an image as an 

 objective of 1 ' 5 mm. without it, the screen or plate remaining at the 

 same distance. 



As the linear magnification of a projected image is the quotient 

 obtained by dividing the distance of the image from the posterior 

 focal point of the objective by the equivalent focal length of the latter, 

 we can determine the magnification at any distance of the image from 

 the eye-piece, by dividing this distance expressed in mm. by the focal 

 length of the objective used, and multiplying the result by the number 

 of the projection eye-piece employed. Thus the objective of 3 mm. 

 gives with the projection eye-piece 2 an image magnified 1000 times 



at a distance of 150 cm., (— — x 2 = 1000 Y This rule holds good 



