160 TESTING THE MICROSCOPE. 



of light is formed by a corresponding but opposite sector of the 

 objective, and consequently shows also traces of the aberration 

 peculiar to the latter. If the objective is under-corrected that is, 

 if the violet rays (v) are more strongly refracted than the red (r) 

 the circle of light has a blue border, and in the case of over- 

 correction a red one, as shown by the construction. 



A precisely analogous proof may be given, that if the Microscope 

 is focused to the lower plane M N', the opposite colours will 

 appear ; a red border indicating under-correction, a blue one, over- 

 correction. 



This method of testing is very simple, and it is theoretically 

 accurate. The former method appears to give more definite results 

 where minute traces of aberration are under consideration. To 

 avoid the somewhat delicate operation of covering up portions of 

 the lenses, a minute aperture in an opaque ground may be adjusted 

 in the centre of the field of view; the mirror is then moved 

 laterally out of the axis of the Microscope ; the incident cone will 

 then meet only that half of the objective which is opposed to the 

 mirror. 



In order to test severely, not merely the halves of the objective, 

 but several of its concentric zones, either singly or variously com- 

 bined, Abbe's method, mentioned above, is most efficient. A dia- 

 phragm having two separate apertures is adjusted so that one of 

 the effective pencils of light in the posterior focal plane of the 

 objective passes centrally through the opening, while the other 

 touches its margin (cf. Fig. 92, right half). The examination of the 

 zones which lie between the centre and the margin is effected by 

 moving the diaphragm from right to left. This method has the 

 additional advantage, that the phenomena of interference produced 

 by small diaphragms are totally eliminated. 



If we test various Microscopes by the method just described, 

 we shall discover a decided under- or over-correction in the 

 majority. Objectives which are achromatic for red and violet rays, 

 and which therefore exhibit the colours of the secondary spectrum 

 only, are, according to our experience, somewhat uncommon. In 

 many Microscopes the coloured borders appear far more intense 

 near the margin of the field of view than in the centre ; examples 

 are found where the aberration observed in one part of the field 

 changes to the opposite aberration on viewing a more central part 

 or the other side of the field. This obtains in a marked degree in 



