ADJUSTMENT OF OBJECT-GLASS. 151 



and when without the focus. If the greater expansion, or coma, 

 is when the object is without the focus, or farthest from the Objec- 

 tive, 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 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 jnresent 

 (like the Podura-scale and the Diatoinacea?) a set of distinct 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 point, the object must be far- 

 ther separated."* "When the Angle of Aperture is very wide, the 

 difference in the aspect of any severe Test under different adjust- 

 ments 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.*}* 



114. Although the most perfect Correction required for each par- 

 ticular object (which depends, not merely upon the thickness of 

 its glass cover, but upon that of the fluid or balsam in which it 

 may be mounted) can only be found by experimental trial ; yet for 

 all ordinary purposes, the following simple method, first devised 

 by Mr. Powell, will suffice. The object-glass, adjusted to ' unco- 

 vered,' is to be 'focussed' to the object ; its screw-collar is next to 

 be turned until the surface of the glass cover comes into focus, as 

 may be perceived by the spots or striae by which it may be marked; 

 the object is then to be again brought into focus by the ' slow 

 motion.' The edge of the screw-collar being now usually graduated, 

 the particular adjustment which any object may have been fo\md 

 to require, and of which a record has been kept, may be made 

 again without any difficulty. By Messrs. Smith and Beck, however, 

 who first introduced this Graduation, a further use is made of it. 

 By experiments such as those described in the last paragraph, the 

 correct adjustment is first found for any particular object, and the 

 number of divisions observed through which the screw-collar must 



* See "Quart. Journ. of Microsc. Science," Vol. ii. (1854), p. 138. 



+ 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, tipon a pin that 

 passes through a longitudinal slit in the outer tube to be attached to the 

 inner. The whole range of adjustment is thus performed within a third 

 part of a revolution, with scarcely any friction, and with such an 

 immediate transition from good to bad definition, that the best point is 

 made readily apparent. 



