ADJUSTMENT OF FOCUS. 163 



tube is furnished, the fine adjustment being otherwise provided 

 for, or being not supplied at all, still greater care is of course 

 required. It is here, perhaps, well to notice, for the guidance of 

 the young Microscopist, that the actual distance between the 

 object-glass and the object, when a distinct image is formed, is 

 always considerably less than the nominal focal length of the 

 object-glass ; thus, the distance of the 1 inch or 2-3 inch object- 

 glass may be little more than half an inch; that of the 4-10 inch 

 may be but little more than one-tenth of an inch ; that of a 1-4 

 or a 1-5 inch may scarcely exceed one- twentieth ; that of a 1-8 

 inch may not be one-fortieth ; and that of a 1-12 or a 1-16 inch 

 may be so close as not to admit the intervention of a piece of 

 glass no more than one-hundredth of an inch in thickness. The 

 reason of this is, that the focal length of an Achromatic objective 

 is estimated by that of the Single lens with which it agrees in 

 the size of the image it forms, and therefore in magnifying power 

 (e. g., it is said to be of 1 inch focus, when its power is equiva- 

 lent to that of a single lens, which brings parallel rays to a point 

 at an inch distance) ; whilst from its being composed of a com- 

 bination of lenses, the point from which that focal distance has 

 really to be measured, is not at the surface of its front lens, but 

 at some distance behind it, in the interior of the combination. 

 One more precaution it may be well to specify ; namely, that 

 either in changing one object for another, or in substituting one 

 objective for another, save when powers of such focal length are 

 employed as to remove all likelihood of injury, the "body" 

 should be turned to one side, where the construction of the Mi- 

 croscope admits of this displacement, or (where it does not) 

 should have its distance from the stage increased by the " coarse 

 movement." This precaution is absolutely necessary, when ob- 

 jectives of short focus are in use, to avoid injury either to the 

 lenses or to the object; and when it is habitually practised with, 

 regard to these, it becomes so much like an "acquired instinct," 

 as to be almost invariably practised in other cases. 



82. In obtaining an exact Focal Adjustment with object- 

 glasses of less than half an inch focus, it will be generally found 

 convenient to employ the fine movement ; and as recourse will 

 frequently be had to its assistance for other purposes also, it is 

 very important that it should be well constructed and in good 

 working order. The points to be particularly looked to in test- 

 ing it, are for the most part the same with those already noticed 

 in relation to the coarse movement. It should work smoothly 

 and equably, producing that graduated alteration of the distance 

 of the object-glass from the object, which it is its special duty to 

 effect, without any jerking or irregularity. It should be so sensi- 

 tive, that any movement of the milled head should at once make 

 its action apparent, by an alteration in the distinctness of the 

 image, when high powers are employed, without any " loss of 



