314 ACCESSORY APPARATUS 



different objectives will with such microscopes prove of no moment 

 if the sub stage is once for all c.-i n-ftil ly fixed centrally in the axis. 



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

 as .seen \\ith a low-power lens through the condenser, so that it 

 -lands 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 mav he added with advantage to this kind of mount, 

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

 scoj e> still so often and so unwisely without condensers this is a 

 most inexpensive and most convenient arrangement. 



An epiiome 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). 



.'!. A tube carrying the optical combination of the condenser 

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



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



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



1 'o/tdensers require special mounting for use ti-ith the polar i 'scope. 

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

 selenites. Swift makes an ingenious inultum in parro mount for 

 'i 11 ploying, amongst other things, the condenser with the polariscope. 

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

 know of 110 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 t he 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. Ac. Xow from the mulct- part of the sub- 

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

 this leaves little to be desired in practice. 



^ ' would ad\ise tiie m icrosco] list to avoid condenser mounts 

 \\hich <-arr\ their own centring movements apart from the sub- 

 stage. It is with regret that we lind that this plan has been adopted 

 in Abbe's ne\\ achromatic condenser. It is manifest Iv better to lit 

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

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

 vluVh requires il, : ,t each piece of sub stage a pparatus which needs 

 centring should be provided with separate fittings tor this purpose 

 <"in have nothing to recommend it. 



! *ecl or usage of m roscopists a diaphragm means a 1ml, 



tjai* PHI! pe g in ih,. illiiplir 



\ ' stop ' is an opaque disc stopping out central < 



