THE OPTICAL EQUIPMENT 



53 



it does not exactly coincide with the plane of the object, the 

 apex of the illuminating cone falls above the object ; the 

 object, in fact, then occupies a position within the solid cone 

 of light itself, and not above the apex of the cone. 



A fairly representative series of sub-stage condensers is made 

 by Messrs. W. Watson & Sons of High Holborn, and may be 

 regarded as sufficiently complete for any class of work and for 

 use with objectives of any power. Most makers now have 

 a series which will fulfil all ordinary requirements, and the table 

 here set out may be taken as representative of what can be 

 obtained in this direction. Messrs. Watson have somewhat 

 specialised in sub-stage condensers, and their list (Table III) 

 probably represents as wide a range of productions in this 

 direction as can at present be obtained. 



TABLE III 



For the illumination of unstained or living objects, some 

 method of dark-ground illumination may be desirable. Such 

 methods depend on the utilisation of a hollow cone of light or 

 annular illumination. To obtain this there are various ap- 

 pliances in use, and some of these date back to the early days 

 of microscopy. The simplest is that known as the ' spot lens,' 

 an arrangement only suitable for low-power work, and having 



