14 MICROSCOPE AND ACCESSORIES. [CII. I. 



screw collar the combinations may be separated, diminishing the power or ap- 

 proximated and thereby increasing it. 



§ 25. Projection Objectives. — These are designed especially for projecting an 

 image on a screen and for photo-micrography. They are characterized by having a 

 Hat, sharp field brilliantly lighted. In power they vary, the lowest being of 75 

 mm. and the highest of 6 mm. equivalent focus (see Ch. IV). 



$ 26. Illuminating or Vertical Illuminating Objectives. — These are designed for 

 the study of opaque objects with good reflecting surfaces, like the rulings on metal 

 bars. The light enters the side of the tube or objective and is reflected vertically 

 downward through the objective and thereby is concentrated upon the object. The 

 object reflects part of the light back into the microscope thus enabling one to see 

 a clear image. 



$ 27. Tube-Length and Thickness of Cover-Glasses. — "In the construction of 

 microscopic objectives, the corrections must be made for the formation of the 

 image at a definite distance, or in other words the tube of the microscope on 

 which the objective is to be used must have a definite length. Consequently the 

 microscopist must know and use this distance or 'microscopical tube-length' to 

 obtain the best results in using any objective in practical work." Unfortunately 

 different opticians have selected different tube-lengths and also different points 

 between which the distance is measured, so that one must know what is meant by 

 the tube-length of each optician whose objectives are used. See table. 



The thickness of cover-glass used on an object (see Ch. VII, on mounting), ex- 

 cept with homogeneous immersion objectives, has a marked effect on the light 

 passing from the object (Fig. 56). To compensate for this the relative positions of 

 the systems composing the objective are different from what they would be if the 

 object were uncovered. Consequently, in non-adjustable objectives some standard 

 thickness of cover-glass is chosen by each optician and the position of the systems 

 arranged accordingly. With such an objective the image of an uncovered object 

 would be less distinct than a covered one, and the same result would follow the use 

 of a cover-glass much too thick. 



