352 



ACCESSOEY APPAKATUS 



however, to their utility that their points should meet accurately, 

 it is well that one of the blades should be furnished with a guide-pin 

 passing through a hole in the other. 



Most microscopists have at some time experienced the danger 

 that is imminent to their instruments and mountings when exhibit- 

 ing delicate objects with high power in mixed assemblies, arising 

 from the inadvertency or want of knowledge 

 of some visitor, who may do terrible mischief 

 by innocently using the coarse adjustment. 

 Messrs. Ross made an arrangement by which 

 the coarse adjustment could be ' locked ' at a 

 given point ; but an equally useful and simpler 

 method was long ago devised by Messrs. 

 Powell and Lealand, who used a deep ring, as 

 is shown in fig. 307. This ring has two pins 

 and a screw projecting inwards. When the 

 screw is withdrawn, the rings can be slipped 

 over the milled heads of the coarse adiust- 



FIG. 307. Powell and Lea- j -, ,-, n1 , 



land's protecting ring for ment, and by screwing the small screw home 

 coarse adjustment. the ring cannot be withdrawn ; but as they 



are loose upon the milled heads, the latter 



cannot be brought into action ; the rings simply revolve upon the 

 heads without bringing them into play. 



Other forms of the same appliance have been made by this firm ; 

 and Messrs. Beck have made these rings with slight modifications 

 more recently. They are the most efficient means of counteracting 

 the danger incident on public exhibition of delicate objects under 

 high powers. 



The foregoing constitute, it is believed, all the most important 

 pieces of apparatus which can be considered in the light of accessories 

 to the microscope. Those which have been contrived to afford 

 facilities for the preparation and mounting of objects will be described 

 in a future chapter (Chapter VI.). 



