158 TESTING THE MICROSCOPE. 



complete coincidence either does not take place at all, or only in 

 one part of the field of view, whilst in other places the corre- 

 sponding lines are seen displaced laterally, or in different planes ; 

 further, in consequence of chromatic aberration, coloured borders 

 appear on the bright lines, which vary according to their position 

 in the field of view. Abbe states that " A test-image of this kind 

 exhibits at one and the same time the whole condition of the cor- 

 rection of a Microscope in all its peculiarities before the eye. By 

 means of the explanation which theory gives for the diagnosis of 

 the different errors of delineation, the comparison of the coloured 

 borders of the single partial images suffices to define exactly their 

 mutual displacements laterally, and their differences of plane, in 

 the central part of the field of view and in the four quadrants of 

 the marginal zone, as well as all faults of correction in the last 

 constituent parts according to their nature and magnitude." 



III. 

 TESTING THE CHKOMATIC ABERRATION. 



THE simplest and most reliable method of testing the chromatic 

 aberration of a system of lenses is to cover one-half of the anterior 

 or posterior surface with black paper or tin-foil, so that only the 

 other half remains optically effective. If we then view a line of 

 light or a small luminous surface for instance, a minute aperture 

 in a blackened plate it will appear colourless in an instrument that 

 is perfectly achromatic ; but in an over- or under-corrected one it 

 will be encircled on the one side by a blue border, on the other by 

 an orange-red or a yellow one. What arrangement these colours 

 will exhibit, depending on the kind of aberration present, may be 

 found without further discussion from Fig. 93. Let A B be the 

 objective to be tested, the right half covered by the diaphragm P. 

 Since the uncovered half acts as a prism, then, if the chromatic 

 aberration is under-corrected, the differently coloured real images 

 of the object a ~b must obviously be so displaced that the violet one 

 (a'v &') projects the farthest on the right, and the red (af r I'*) on 

 the left. The microscopic image has, therefore, on the right side 



