OP THE MICROSCOPE. 67 



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 fie 

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

 ation will be shown by the lines being curved. 



"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 which 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 Jitil field. Want of flat- 

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

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

 general it happens taafc 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, and few scientific disputes have been waged with more 

 bitterness and personalities than that which has been called the 

 "Battle of the High and Low Angles." Now that the smoke 



as in a measure cleared away, and that we are able to take a 



