26 Modern Microscopy. 



Now as to the Fine Adjustment. A great diversity of 

 opinion is existent with regard to the qualities of the 

 different forms, most writers having their preferences and 

 prejudices. It is a most important movement, and it is 

 compulsory that it shall be quite perfect if high powers are 

 to be used. Personally, after some years of practical 

 acquaintance with Messrs. Watson and Sons' instruments, 

 I can say that with high powers of .large aperture I 

 have found their form of fine adjustment perfectly satis- 

 factory. It is shown in position on the instruments (Figs. 

 1 and 2), and consists of a milled head with a micrometer 

 screw actuating a lever on a pin fixed to the body against 

 a long spiral spring, whereby the body is raised or de- 

 pressed in a dovetailed fitting. One arm of the lever is 

 four and a half times longer than the other, whereby the 

 weight of the body at the milled-head end of the lever 

 and the motion imparted is reduced in this ratio. Thus 

 the makers give the weight of a body as 17 oz., and 

 this divided by 4J reduces the resistance to 3 oz. This 

 system has many advantages. The position on the limb 

 is convenient for manipulation, and is not altered when 

 the body is racked up that is, it is not carried by the rack- 

 work as in many forms, so that its attachment to a focussing 

 rod of a camera for photo-micrography is easy and con- 

 venient. There is also a very simple means of adjustment 

 provided for taking up any slackness through wear. The 

 slide in which the fine adjustment is fitted has sprung slots, 

 to which are fitted screws (shown in Fig. 3, page 24, 

 marked A). By turning these screws slightly, the spring 

 fitting grips the bearing more tightly, and so takes up any 

 wear through friction. Any microscopist can therefore 

 adjust his own instrument. 



Another very good and rigid form is that fitted by Messrs. 

 Swift and Son. It consists of a lever in a vertical direction 

 raising and lowering the body tube. The actuating milled 

 head is shown in position on the instrument on page 29, 

 marked ' patent,' and is carried by the coarse adjustment 



