The Microscope. *> 



objective which will resolve P. angulation nicely with central 

 light, when the mirror is exactly focused. By moving the lat- 

 ter out of focus it will be seen that the objective looses in per- 

 formance, and if this is carried sufficiently far it will arrive at 

 a point where the objective will cease to show any lines. The 

 effect will be the same on any other object, and is caused by 

 the lack of proper concentration of light on the slide. When 

 oblique light is used, unless the diaphragm moves with the 

 mirror, it should be removed, as the advantage of obliquity is 

 diminished or destroyed by the loss of light. 



The cover-glass exerts probably the greatest influence in 

 testing as well as in general work. This should be used of a 

 thickness which corresponds to that to which the objective (if 

 non-adjustable) was originally corrected. If thicker or thinner 

 covers be used, the objective will be spherically over or under 

 corrected, and will have to be moved correspondingly above or 

 below the plane (outline) of the object to distinguish its struc- 

 ture, if the variation is considerable the difference between the 

 two planes will be so great that it will cease to show any struc- 

 ture, and it may then be said to be lacking in defining power 

 although in reality it possesses it, but is not properly used. 

 Generally speaking the objective may be said to be spherically 

 corrected when it gives the best defined image; that is, when 

 the outline and internal structure of an object of extreme 

 thinness appears in one plane. When, after the objective has 

 been focused on the outline of the object, it is necessary to in- 

 crease the distance to focus on the structure, it is evidence that 

 the objective is spherically over-corrected and that the cover is 

 too thick ; in an adjustable objective, the correction collar must 

 be brought to its closing point, which means that the lenses are 

 brought in closer contact. When the objective must be 

 focused to a point beyond the outline of the object to see its 

 structure ; that is, brought closer to the cover-glass, it nroves 

 that this is too thin, and is then called spherical under-correc- 

 tion ; to give the proper adjustment in an adjustable objective 

 in this case the adjustment is opened— the lenses are separated. 

 It requires a certain amount of study to distinguish these phe- 

 nomena, and although it can be done in well prepared speci- 

 mens, I know of none better than coarsely marked diatoms, 

 such as P. angulatum. 



