CHANGING NOSE-PIECES 393 



Zeiss ; this is so arranged that only the optical portion of the objec- 

 tive is screwed into the nose-piece. This plan much lightens it, so 

 that the nose-piece and the four lenses weigh 3| oz., or only 1 oz. 

 more than an English J-inch with a screw collar, and ^ oz. more than 

 an English ^-inch of wide angle. 



A centring nose-piece has been made with the view of placing 

 any objective central to the axis of rotation of the stage. It is, of 

 course, much cheaper to centre an objective by means of a nose-piece 

 to the axis of rotation of the stage than to centre the rotary stage 

 to the objective. This, like all other adapters, is an additional 

 weight ; but here there is very little to be gained by it, for if the 

 rotary stage is well made any objective will be sufficiently centred 

 for all practical purposes. Mr. Nelson, as we have seen, pointed 

 out, at a time when the sub-stage was costly, that such a nose- 

 piece turned upside down, with a,turn-out rotating ring for stops, &c., 

 fitted below, made a very efficient rectangular centring sub-stage 

 at a small cost. Sub-stages are now quite common and cheap, and 

 centring nose-pieces are seldom used for any purpose. 



Next to the rotating, probably the changing nose-piece is the 

 most important. We do not know from whom, and when, the idea 

 of an arrangement by which an objective could be rapidly attached 

 or detached originated ; but certain it is that the idea is admirable, 

 and one which is scarcely yet as fully appreciated as it should be. 

 It will be quite impossible to go through a tithe of the appliances 

 which have been invented for this purpose ; it will be sufficient to 

 lay down some principles, and mention a few in which those prin- 

 ciples are fulfilled. 



The first principle is that the objective or nose-piece, adapter, 

 or whatever else is used, should ' face up.' This means that a flange 

 turned true in the lathe should ' face up ' to the flat side of the nose- 

 piece, which has also been turned true. This ' facing up ' should be 

 made tight by a screw, inclined plane, or wedge, itc. Unless this 

 is done you have no guarantee that the axis of the objective is 

 parallel to that of the body. Therefore all those appliances which 

 merely grip the objective, or an adapter screwed on to the objective, 

 are simply of no value. Secondly, the appliance, whatever it is, 

 should be light. 



Xachet's changing nose-piece, which fulfils none of these con- 

 ditions, cannot be called good. The nose-piece is large and heavy, 

 even for the small objective it is intended to take, the screws of 

 which are -/ only in diameter, against the |-| of that of the Society. 

 The objectives are held by a spring clip on a small flange. Of course, 

 screw-collar adjustment with such a device would be simply im- 

 possible. Zeiss's sliding-objective changer is most elaborate and 

 efficient, although, as w T e think, much heavier than it need be. It 

 consists of a grooved slide which screws on to the nose-piece. On 

 each objective is screwed an adapter to slide into the grooved nose- 

 piece. These adapters, which are wedge-shaped and i face up/ have 

 two novel features, the first being that they are each fitted with 

 rectangular centring adjustments, which permit the objectives 

 to be centred to one another ; and the second is that they have 



