170 TESTING THE MICROSCOPE. 



Y. 



TESTING THE CENTERING. 



WE have shown that imperfect centering of a system of lenses 

 manifests itself generally in two ways : (1) by producing indistinct- 

 ness in the images, which first affects the margin of the field of 

 view, and only in the case of greater deviations extends to the 

 centre in all cases, however, decreasing towards the centre ; (2) 

 by the changes of position which any fixed image-point undergoes 

 if the objective or one of its component lenses is rotated on the 

 optic axis. The first point may be tested by determining the 

 optical power of the Microscope for different positions of the field 

 of view, and comparing the results. It must be specially noticed 

 whether the left margin gives decidedly more indistinct images 

 than the right, and whether there is inequality in the distribution 

 of optical power, and in its decrease from the centre towards the 

 periphery. If these differences are not noticeable, or but a slight 

 loss towards the margin can be distinguished, conditioned by the 

 aberrations of the eye-piece and therefore proportional to them, 

 we may be sure the errors of centering are as far as possible 

 corrected, i.e., are reduced to zones which are situated outside the 

 field of view, or project only very slightly over the borders. 



Regarding the second point, it is usually stated in micrographic 

 works, that the changes of position in question are apparent on 

 revolution of the objective-lenses by unscrewing the mounting; 

 it is therefore only necessary to adjust the Microscope on any 

 small object, so that the image may touch the margin of the field 

 of view or a line of the eye-piece micrometer ; imperfect centering 

 would then be shown, on unscrewing the lenses, by a corresponding 

 circular movement of the microscopic image. 



We mentioned this method of testing in our first (German) 

 edition, adding further details with regard to the factors upon 

 which the amount of the displacement is dependent. We then 

 stated that "the movements of the image observed during the 

 revolution of the objective lenses by unscrewing are dependent 

 upon the accuracy of the construction of the mountings and screw- 

 threads quite as much as upon the deviations of the optic axes 



