286 TUBE-LENGTH AND COVER-GLASS THICKNESS [Cn IX. 



glass. If the lenses are fixed in position in the objective it will 

 be evident that one must select a cover-glass which is of the exact 

 thickness to satisfy the correction of the objective. The makers of 

 objectives are now very precise in stating exactly how thick the 

 covers should be for their objectives, and it is the part of wisdom 

 to pay heed to their statements if one hopes to get the best re- 

 sults. 



If one's objectives are adjustable ( 134-135), it is possible to so 

 arrange the combinations that quite a range of cover-glass thickness 



Slide 



I 



FIG. 169. SPHERICAL ABERRATION INTRODUCED BY THE COVER-GLASS. 



Axis The principal optic axis extending through the condenser and up through 

 the object and microscope. 



Slide The glass slide on which the object is mounted. 



Object The object to be studied; it is mounted on the slide. 



Balsam The medium in which the object is mounted. It has practically the 

 same refractive index as the cover. 



Cover-glass The thin glass plate over the object. 



i 2 3 The light rays extending obliquely upward from the object. 



3 2 i Light rays traced backward to their apparent origin, the most oblique 

 ray (j) being most bent, thus rendering its origin apparently highest. 



r r r Points of refraction of the three oblique rays. 



or mounting medium thickness can be used and still get the 'best 

 optical effect by balancing the aberrations (462). 



461. Tube-length. The length of the tube on the microscope 

 must be made of the standard for which the objective used was cor- 

 rected or aberrations will appear. 



If the tube is shorter than the objective was corrected for, the 



