Arrangements for Focussing. 27 



when focussing. This is rather a disadvantage, especially 

 in photo-micrography, as it means altering the position of 

 the connecting arrangements of the camera for different 

 powers ; also, being on the side of the instrument makes 

 it additionally awkward, as only one hand can be applied 

 to it ; Watson's form in the constant position being more 

 convenient on this account. For their working qualities 

 both of the fine adjustments mentioned are very good in 

 fact, as perfect as can be desired but for convenience 

 Watson's is preferable. Powell and Lealand's instruments 

 are also provided with a fine adjustment having special 

 merit, consisting of a lever actuating a long tube sliding up 

 and down inside the body. This, again, has the disadvan- 

 tage of being carried by the rackwork when focussing for 

 different powers. The differential screw fine adjustment, 

 as made by Mr. Charles Baker, has been well spoken of, 

 but the writer has had very little experience with it. 



Nearly every maker has his own system of fine adjust- 

 ment, possessing features more or less desirable, but they 

 are mostly modifications of those mentioned here. Above 

 all things eschew the form of fine adjustment which carries 

 the whole weight of the body of the instrument, or depresses 

 it against a spring, as in the Continental instruments and 

 cheap students' forms : these are almost worse than no fine 

 adjustment at all, as they invariably soon work loose in the 

 fittings and cause great annoyance. 



There is also another form, now almost non-existent, 

 which is fitted on to the microscope at the nosepiece end, 

 and consists of a milled head attached to the body moving 

 a tube inside the body-tube against a spring. This cannot 

 ever work thoroughly well, as, if two round tubes are 

 perfectly true and fit well they clutch, and to overcome this 

 one of the tubes is rendered a little eccentric, giving a 

 lateral shake. 



In the choice of a fine adjustment, therefore, eschew the 

 direct acting and the nosepiece forms. 



