1 62 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



racked down ; so that if the racks are properly placed relatively to 

 one another ' loss of time ' is impossible ; and the result is obtained 

 without forcing the teeth of the pinion into the rack. If the teeth 

 are true, the friction is of the least, and the smoothness and firm- 

 ness all that can be desired. But what gives great value to this 

 form of rack is that any loss of time as the result of wear can be 

 taken up by a slight alteration of the position of the second rack. 

 The arrangement is shown in fig. 124 A, and it will be seen ,that at 

 the top of the right-hand rack as we look at the illustration there is 

 a small screw. Now the racks are set side by side, one being fixed 

 finally. The pinion is then made to work freely and smoothly with 

 this one rack ; the second rack is then introduced, and is provided 

 with slots and clamping screws, and its position is gradually altered 

 in the slots in a vertical direction by means of this small screw over 

 the right-hand rack until the smoothest position of action is secured. 

 The clamping screws are then tightened and the rackwork becomes 

 fixed ; and subsequent irregularity in it is at once corrected by the 

 small screw to which we have referred. 



When the best position is found the teeth of the two racks, MS 

 we have stated, will not be in a line, but those of the loose rack will be 

 found to occupy a position slightly below the teeth of the fixed 

 one. 



There is a defect in either microscope or microscopist if the 

 ' fine adjustment ' is resorted to before the object is focussed into 

 clear view, even with the highest powers. 



The Fine Adjustment. This part of the modern microscope 

 possesses an importance not easily exaggerated, and deficiency or 

 bad principle in the construction of this makes not only inferior, 

 but for critical purposes absolutely useless, what are otherwise 

 instruments of excellent workmanship and real value. 



There are two kinds of fine adjustment usually employed : 



i. Those which simply move the nose-piece which receives the 

 objective. 



ii. Those which move the whole body, or the whole body including 

 the coarse adjustment. 



All constructions of the second class formerly proved impracti- 

 cable, and even pernicious. They inevitably broke down just as the 

 purchaser, by practice, began to realise the value of perfect action. 

 With a large experience of stands of every class, we are obliged to 

 say that generally with one or two years of work they lost whatever 

 value they at first possessed. 



To this broad statement there are possibly two or three excep- 

 tions, viz. Swift's side lever and Campbell's differential l screw and 

 Watson's long lever, to which we shall subsequently refer. 



It is, however, upon the model above referred to, with all its 

 radical and glaring imperfections, that the majority of Continental 

 microscopes have been built. 



A screw with an extremely fine thread, and therefore of extremely 

 shallow incision a micrometer screw in fact has to bear the strain of 



1 The differential screw fine adjustment was first suggested by Dr. Goring in 

 1830. It was subsequently made by Nobert about 1865. 



