100 THE MICROSCOPE 



an iris diaphragm. In the upper cell of this a small Abbe con- 

 denser can be fitted, and also a series of patch-stops or a coloured 

 or ground glass. The fitting can be moved up and down in the 

 tube to a limited extent for focussing. 



No. 3211. No. 3211 (page 98) has a substage which focusses by means of 



a screw actuated by a milled knob. When this screw reaches the 

 end of its travel in a downward direction the whole fitting carrying 

 the iris diaphragm and condenser swings aside, so that it is a 

 very simple matter to entirely dispense with the condenser 

 when it is not required. The substage is held rigidly in the 

 optic axis imtil the fitting is focussed down to its lowest position, 

 when a further turn of the milled head swings it out of position, 

 thus the addition of this motion does not in any way afiect the 

 rigidity of the substage. This substage is suitable for use with 

 either the small or large form of the Abbe condenser. Centring 

 motions cannot be fitted, and it is consequently not suitable 

 for use with an achromatic condenser. A high-power dark- 

 ground illuminator can be used with it, but must be in a fitting 

 that is provided with centring screws. 



No. 3213. No. 3213 (page 99). This microscope has a substage with full 



adjustments — namely, focussing by rack and pinion, swing-out 

 motion, and centring motion. The focussing is actuated by a large 

 milled head on the right of the instrument travelling in the same 

 way as the coarse adjustment milled heads. When at the bottom 

 of its travel the substage may be completely swung aside. Here 

 again, as this substage is held in position by a guiding pin, until 

 it is in its lowest position, there is no tendency to lose rigidity 

 by the addition of the swing- out motion. The centring is actuated 

 by two screws with milled heads. A modified form of this stand. 

 No. 3212, is made which is the same as No. 3213, except that the 

 substage has not a centring adjustment. This is suitable when 

 it is not desired to use a higher class condenser than the Abbe 

 form, but for all more exacting work the No. 3213 is preferable. The 

 substage on this stand No. 2313 enables the achromatic condenser 

 and the high-power dark-ground illuminator to work to their 

 full advantage. The illustration, page 99, shows this microscope 

 with a detachable mechanical stage attached by bolt and nut, 

 as mentioned previously. 



The Portable Standard London Microscope 



Portable This microscope has been designed for the use of the micro- 



No. 3221. ' scopist whose work requires that he should have an instrument 

 of the usual rigid construction, with all the movements neces- 

 sary for the highest forms of research work, but to whom porta- 

 bility is also an advantage. For travellers engaged in critical 

 work, and bacteriologists in foreign countries, this microscope is 

 especially suitable. The stand is the same as the standard 



