228 



THE AMERICAN MONTHLY 



[December, 



of condenser affords an angular ajer- 

 ture, according to the maker's state- 

 ment, of 90° or 180°, as may be desired. 



Passing on to the mechanical de- 

 vices, it will be seen that immediately 

 beneath the optical part there is a 

 thin revolving plate E with circular 

 apertures, in which may be placed 

 stops of various kinds for modifying 

 the illumination, and films of selenite 

 fcr use with the polariscope. This 

 plate is readily turned by applying the 

 finger at E, bringing the different stops 

 successively into use. The condenser 

 is centred by the substage-screws C C 

 w 01 king against a single spring. 



Immediately cbove the revolving 

 plate, movirg quite independently of 

 ir, aie two plate holders, one of which 

 is diown at D. These carry films of 

 mica, which, when used in combina- 

 tion with the selenites in polarized 

 light, give a great variety of colored 

 effects. On the periphery of D are 

 fine teeth which engage with other 

 teeth of the corresponding plate-holder, 

 which is not shown in the figure, be- 

 ing hidden frcm view beneath the 

 condensing lenses. By applying the 



Fig. 44. — Swift's Mounting for Condenser. 



finger at Z> the outer plate is revolved 

 and this causes the other plate to 

 turn in the opposite direction. Either 

 of the plates can be brought into use 

 at will. 



'I'he polarizing prism F is perma- 

 nently attached beneath the substage, 

 swinging out and in on an arm, so that 

 it can be instantly brought into use. 

 A form of iris diaphragm is fitted be- 

 neath the condenser, adjusted by the 

 lever shewn at B. 



A most convenient feature of this 

 apparatus is the method of attach- 

 ment beneath the stage. In fig. 43 it 

 is represented with a rack, as it was 

 originally made. Now, however, the 

 condenser is fitted into a centering 

 substage such as is shown in fig. 44, 

 which fits into horizontal grooves in 

 the microscope, so as to be readily re- 

 moved or replaced at will. By this 

 means the rack and pinion, substage, 

 and condenser are all removed in one 

 piece, leaving the entire space from 

 stage to mirror free. 



In thus according high praise to this 

 condenser, it may be asked is it su- 

 perior to that of Prof. Abbe ? We 

 would reply, it is more universal in 

 its application, and for that reason 

 better for the ordinary microscopist. 

 On the other hand, it does not give 

 so great an angle of illumination as 

 the Abbe form, but the lenses of the 

 latter can be used instead of the com- 

 binaticn of Mr. Sv\ift, without any 

 change in the mechanical arrange- 

 ments. The condenser may be readily 

 applied to any microscope of sufficient 

 size, but it must be specially fitted. 

 It is rather expensive, but it answers 

 the purpose of condenser, spot-l:ns 

 or paraboloid, and polarizer, and is 

 not, therefore, more costly than the 

 several appliances for which it forms a 

 substitute. 



Another ingenious contrivance ap 

 plied to some of Mr. Swift's stands is 

 precisely what we have endeavored 

 to induce our own manufacturers to 

 adopt, but without success. It is a 

 levolViUg plate of diaphragms fitted in 

 the stage-plate. This is shown in the il- 

 lustration (fig. 45), of the modification 

 of the stand first introduced by Mr. 

 George Wale, made by Messrs. Swift 

 & Son, under the name of the "im- 

 proved Wale's American Microscope." 

 The milled edge of the diaphragm- 

 plate is seen projecting from the front 

 of the stage-plate. The device is not 

 new, ben-g in fact, as we are told, a 

 very old one ; but it is none the less 

 good, and Messrs. Swift & Son apply 

 it to several of their microscopes. 



