146 THE MICROSCOPICAL NEWS, 
TESTING MICROSCOPE OBJECTIVES? 
By Dr. ALLEN Y. Moore. 
JITH all the microscopical literature we have, it is surprising 
to see how little attention objectives have received, and, at 
the same time nearly every work on the subject tells us that the 
objective is the most important part of the entire instrument. ‘That 
this is a fact, I think no person will deny, and if the objective be 
otherwise than perfect, a knowledge of its imperfections will help 
the observer by giving him an idea of how far his instrument may 
be considered trustworthy. 
All first-class objectives in which the angular aperture exceeds 
75 or 80° in air, should have a correcting collar; and although it 
makes less difference with the lower powers, the higher ones should 
be so constructed that the collar moves the back system of lenses. 
Bad centering and bad form are conditions which are inexcus- 
able. By bad centering is meant, that the centres of the lenses are 
not in a straight line. By bad form is meant that the curves of the 
lenses are not segments of true spheres. Either of these conditions 
may be detected by what is known as the artificial star. ‘This is 
nothing more than a very small globule of mercury upon which a 
strong light is thrown. 
If such a globule be placed in the centre of the field and carefully 
focussed, a bright spot, which is the image of the source of light, 
will be seen. If the tube be now racked up or down, so as to throw 
the globule considerably out of focus, it will be seen that the bright 
spot expands and becomes a large circle of light. If the lenses be 
truly formed and correctly centered the edge of this circle, or ex- 
pansion of light, will be true and even, but in case the lenses are 
not in a straight line, the circle of light, or coma as it is more 
frequently called, will not be perfectly round but will be seen to 
bulge more on one side than on the other. Bad form is shown by 
an irregularity of the outline of the coma, and the shape of the 
coma will indicate the shape of the curved surfaces of the lenses. 
Chromatic and spherical aberration next deserve attention. 
Chromatic aberration is that condition in which the white light, 
after having been decomposed by the refracting surfaces of the 
crown lenses, is not properly recomposed by the proper action of 
the flints. When an objective brings the violet light to a focus 
before the red, it is said to be chromatically undercorrected ; if the 
red rays meet sooner than the violet, it is said to be overcorrected. 
* Read before the Griffith Club of Microscopy, of Detriot, February 6th, 1883. 
