166 MANAGEMENT OP THE MICROSCOPE. 



such proximity with the other two, as to produce a " positive 

 aberration" in the objective, fitted to neutralize the "negative 

 aberration" produced by the interposition of a glass cover of a 

 certain thickness. It is evident, however, that unless the par- 

 ticular thickness of glass for which this degree of alteration is 

 suited, be always employed for this purpose, the correction can- 

 not be exact ; and means must be taken for adapting it to every 

 grade of thickness, which may be likely to present itself in the 

 glass covers. Unless this correction be made with the greatest 

 precision, the enlargement of the angle of aperture, to which our 

 Opticians have of late applied themselves with such remarkable 

 success, becomes worse than useless ; being a source of dimi- 

 nished instead of increased distinctness in the details of the ob- 

 ject, which are far better seen with an objective of greatly inferior 

 aperture, possessing no special adjustment for the thickness of 

 the glass. . The following general rule is given by Mr. Wenham, 

 for securing the most efficient performance of an object-glass 

 with any ordinary object: " Select any dark speck or opaque 

 portion of the object, and bring the outline into perfect focus ; 

 then lay the finger on the milled head of the fine motion, and 

 move it briskly backwards and forwards in both directions from 

 the first position. Observe the expansion of the dark outline of 

 the object, both when within, and when without, the focus. If 

 the greater expansion, or coma, is when the object is without the 

 focus, or furthest from the objective, the lenses must be placed 

 further asunder, or towards the mark ' uncovered.' If the 

 greater coma is when the object is within the focus, or nearest to 

 the objective, the lenses must be brought closer together, or 

 towards the mark < covered.' "When the object-glass is in proper 

 adjustment, the expansion of the outline is exactly the same both 

 within and without the focus." A different indication, however, 

 is afforded by such "test-objects" as present (like the Podura- 

 scale and the Diatomacese) a set of distinct dots or other mark- 

 ings. For " if the dots have a tendency to run into lines when 

 the object is placed without the focus, the glasses must be brought 

 closer together ; on the contrary, if the lines appear when the 

 object is within the focal point, the object must be further sepa- 

 rated. 1 When the angle of aperture is very wide, the difference 

 in the aspect of any severe test under different adjustments be- 

 comes at once evident ; markings which are very distinct when 

 the correction has been exactly made, disappearing almost 

 instantaneously when the screw-collar is turned a little way 

 round." 2 



1 See " Quart. Journ. of Microsc. Science," vol. ii, p. 138. 



2 Mr. Wenham remarks (loc. cit.), not without justice, upon the difficulty of making 

 this adjustment, even in the Objectives of our best Opticians; and he states that he has 

 himself succeeded much better, by making the outer tube the fixture, and by making 

 the tube that carries the other pairs slide within this ; the motion being given by the 

 action of an inclined slit in the revolving collar, upon a pin that passes through a longi- 

 tudinal slit in the outer tube, to be attached to the inner. The whole range of adjust- 



