292 ACCESSOEY APPARATUS 



treated under the heading of ' Microscope/ no more will be said at 

 present than that a double nose-piece is to be preferred to a triple, 

 and a quadruple need not be entertained for a delicate instrument 

 when made of ordinary metal, unless it is required to find out in how 

 short a time a fine adjustment may be ruined ; for let it be noted 

 that a 2-inch, 1-inch, J-inch, and J-inch objective of English make 

 weigh together 8J oz. without any nose-piece. But Messrs. Watson 

 and Son have devised and made in aluminium a dust-proof triple 

 nose-piece, which, where it is required to be used, reduces the objec- 

 tions to its employment to their minimum, and not only in greatly 

 reduced weight, but in other ways, makes its use more feasible 

 without strain upon the fine adjustment or danger of injury to the 

 objectives. In many nose-pieces, if the objectives should be acci- 

 dentally left so that neither of them is in the optical axis of the 

 microscope, there is nothing to guard the back lenses of the objec- 

 tives from dust and moisture. Messrs. Watson devised a dust- 



FIG. 234. Watson's dust-proof aluminium nose-piece. 



FIG. 235. Section of the above. 



proof arrangement, consisting of an upper and an under disc, having 

 a spherical curve ; to the lower disc are fitted three small screw 

 tubes which receive the objectives. This plate rotates upon a centre 

 pin, and as each objective is brought into the optical axis of the 

 microscope its axial coincidence is indicated by a spring catch. The 

 edge is covered with a metal rim, making it dust-proof. The weight 

 of the ordinary brass nose-piece is 4| oz. ; the weight of this one is 

 lj oz. Similar instruments are made by other makers, but the 

 dust-proof arrangement and the extreme lightness are, so far as we 

 know, characteristic of the instrument of Messrs. Watson. We 

 illustrate this nose-piece complete in fig. 234, and in an enlarged 

 section in fig. 235. 



For the proper use of a rotating nose-piece the length of the 

 objective mounts should be so arranged that when the objective is 

 changed little focal adjustment will be necessary. 



An excellent calotte nose-piece for four objectives is made by 



