314 ACCESSORY APPARATUS 



different objectives will with such microscopes prove of no moment 

 if the sub-stage is once for all carefully fixed centrally in the axis. 



What we require to do is to centre the image of the lamp flame, 

 as seen with a low-power lens through the condenser, so that it 

 stands in the middle of the field. This can be done by moving the 

 lamp or the mirror, and until this is satisfactory the best results 

 cannot be obtained. To obviate the inconvenience of having to re- 

 move the combination in order to alter a diaphragm l or stop in 

 this simple mount an internal sliding tube may be used. It will be 

 a further advantage to have a separate cell to fit into the bottom of 

 the sliding tube to receive coloured glasses ; a spiral slot-focussing 

 arrangement may be added with advantage to this kind of mount, 

 acting like a pocket pencil. For students' arid elementary micro- 

 scopes still so often and so unwisely without condensers this is a 

 most inexpensive and most convenient arrangement. 



An epitome of its principal points may be of service. 



1. A sub-stage tube fixed centrally to the body of the microscope. 



2. A spiral slotted tube to push into (1). 



3. A tube carrying the optical combination of the condenser 

 sliding into (2), with a pin moving in the spiral slot. 



4. A long tube carrying the diaphragm and slots sliding into (3). 



5. A cell carrying coloured glasses sliding into the bottom of (4). 

 Condensers require special mounting for use with the polariscope. 



Then at least two ' turn-out ' rotating rings are required to hold 

 selenites. Swift makes an ingenious multum in parvo mount for 

 employing, amongst other things, the condenser with the polariscope, 

 to which we call attention in describing the polariscope. But we 

 know of no plan equal to that found in the best stand of Powell and 

 Lealand. The sub-stage has a double ring, one placed concentrically 

 within the other. The inner one revolves by a milled head and 

 receives the usual sub-stage apparatus. The outer one receives a 

 mount of three selenites which revolve, and are placed on ' turn-out ' 

 arms. On the upper part of this mount of selenites is a screw, which 

 receives the optical combination of their dry achromatic condenser. 

 When this is screwed in its place we have a condenser of the first 

 order, with a mount of three plates of selenites taking the place of a 

 mount of diaphragms, &c. Now from the under part of the sub- 

 stage into the inner and revolving ring is fitted the polariser, and 

 this leaves little to be desired in practice. 



We would advise the microscopist to avoid condenser mounts 

 which carry their own centring movements apart from the suit- 

 stage. It is with regret that we find tha,t this plan has been adopted 

 in Abbe's new achromatic condenser. It is manifestly better to fit 

 the rectangular movements to the sub-stage, and then they become 

 available for all the apparatus employed with the sub-stage. A plan 

 which requires that each piece of sub-stage apparatus which needs 

 centring should be provided with separate fittings for this purpose 

 can have nothing to recommend it. 



1 In the technical language or usage of microscopists a diaphragm means a hole 

 or aperture ; thus a ' large diaphragm ' means that the opening in the diaphragm 

 plate, disc, or iris is large. A ' stop ' is an opaque disc stopping out central rays. 



