The Able Test-Plate 73 



such, for instance, as will be afforded by the Holoscopic 

 eyepiece described on page 80. Those who have not such 

 a convenience may unscrew the eye lens of the Huyghenian 

 eyepiece and so secure some slight modification of correc- 

 tion. The object of this is to ascertain whether the lines 

 may be rendered free from coloured edges or with the same 

 colour on both edges all over the field. If in this pre- 

 liminary step it be found that the definition is unsatisfac- 

 tory, thicker and thinner cover-glasses should be tried ; and 

 in the event of failure to secure good definition in this way, 

 and no reasonable alteration of tube-length will produce 

 the desired effect, the objective may be safely rejected 

 as bad. 



It probably will be found that under one of the cover- 

 glasses the lines will appear satisfactorily defined, in which 

 case the centring may be examined. 



DEFECTIVE CENTRING shows itself (a) by the impossibility 

 of removing the coloured edges of the lines all over the 

 field even when the eyepiece is adjusted as described above, 

 the edge colouring being more apparent on one side of the 

 field than the other ; (b) by unequal definition of the two 

 edges of the central lines, one edge appearing sharp or 

 nearly so, while the other edge is seen double or foggy. 



SPHERICAL ABERRATION. The fact that the objective will 

 bear high-power eyepieces on the test-plate in a satisfactory 

 manner is in itself proof of good correction in this respect, 

 but the following is a further excellent and convincing test : 



Place a diaphragm beneath the condenser having an 

 aperture that will cause the condenser to yield a cone of 

 illumination equal to one-fourth the N.A. of the objective 

 under examination, and while observing the lines change 

 the position of this diaphragm from central to extremely 

 oblique, the obliquity being in a plane at right angles to the 

 direction of the lines. This is best performed by means 

 of one of the mechanical condenser carriers, such as are 

 provided in the Continental microscopes, one of which is 

 figured in the microscope on page 12. If the lines remain 



