OF THE MICROSCOPE. 4:7 



from which the illumination comes. The consequence is, that 

 an objective which may give excellent results on one stand, 

 may fail on another. An easy way of testing this fact, is by 

 means of a rotating adapter. Of course the best test for aber- 

 ration of form is the artificial star, though in the hands of the 

 beginner, a micrometer, ruled into squares, is probably the 

 most available test. Any trace of the defect under consider- 

 ation will be shown by the lines being carved. 



When the lines appear curved, from the fact that the spherical 

 aberration has not been properly corrected, the nature of the 

 error may be determined as follows: "When the micrometer 

 lines are widest apart at the centre (like the lines on a map of a 

 hemisphere) the spherical aberration has been over-corrected. 

 It is under-corrected when the reverse is the case. 



Aberration of form is one of the worst faults with which a 

 lens can be affected, and experience has shown us that it is the 

 one ^vhich is least apt to be detected by a beginner. An objec- 

 tive may give a "beautiful" image, and yet be worthless 

 because affected with this defect. 



Flatness of Field. If, when we examine a perfectly flat 

 object, every part included in the field of view is clearly in 

 focus, the objective is said to have a flat field. Want of flat- 

 ness of field is shown by some parts of a flit object being 

 clear and well-defined, while other parts are out of focus. In 

 general it happens that where this defect exists, the centre and 

 circumference of the objective do not act together. 



Angular Aperture. This subject has given rise to some 

 of the most vexatious questions connected with microscopy, for 

 a discussion of which we must refer our readers to the pages of 

 the microscopical journals published during the past few years. 

 The views which have been promulgated by the two schools into 

 which microscopists have been divided on the questions affect- 

 ing angular aperture, have been of an extremely opposite 

 nature. Thus, the Boston school claim to have produced ob- 

 jectives whose angle of aperture is 180, while the English 

 microscopists ridicule any such claims. Moreover, while what 

 may be called the English school lay it down as a law that the 



