14:0 THE MICROSCOPE AND ITS REVELATIONS. 



part of the outer tube an oblong slit is made, as seen at D, into which 

 projects a small tongue screwed on the inner tube; at the side of the 

 former two horizontal lines are engraved, one pointing to the word ( un- 

 covered/ the other to the word i covered;' whilst the latter is crossed by 

 a horizontal mark, which is brought to coincide with either of the two 

 lines by the rotation of the screw-collar, whereby the outer tube is moved 

 up or down. When the mark has been made to point to the line ' uncov- 

 ered/ it indicates that the distance of the lenses of the object-glass is 

 such as to make it suitable for viewing an object without any interference 

 from thin glass; when, on the other hand, the mark has been brought by 

 the revolution of the screw-collar into coincidence with the line ' cov- 

 ered/ it indicates that the front lens has been brought into such proxi- 

 mity 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 extremest thickness. But unless this cor- 

 rection be made, with the greatest precision, to the thickness of the par- 

 ticular cover in use, the enlargement of the Angle of Aperture, to which 

 Opticians have of late applied themselves with such remarkabe success, 

 becomes worse than useless; being a source of diminished instead of in- 

 creased distinctness in the details of the object, which are far better seen 

 with an Objective of greatly inferior aperture, possessing no special ad- 

 justment 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. Ob- 

 serve 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 farthest from the Objective, the lenses 

 must be placed farther 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 ex- 



Eansion of the outline is exactly the same both within and without the 

 3cus." A different indication, however, is aiforded by such ' test-ob- 

 jects ' as present (like the Podura-scale and the Diatomacese) a set of dis- 

 tinct dots or other markings. 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 poinb, the lenses must be farther separated." * 

 When the Angle of Aperture is very wide, the difference in the aspect of 

 any severe test under different adjustments becomes 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. (1854), 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 longitudinal slit in the outer tube, to be attached to the 

 inner. The admirable Objectives in the first-class American Opticians, are (the 

 Author believes) always constructed so that the adjustment is effected by the 

 movement of the back combinations, as long since recommended by Mr. Wenham. 



