12 



THE AMERICAN MONTHLY 



[January, 



hundredths of an inch, attached to 

 the limb where it joins the tube. 



With a magnifying power of about 

 2,000 diameters used on this stand, 

 there is not the slighest apparent dis- 

 placement of the object by any motion 

 of the fine adjustment, and the work- 

 ing of the screws-collar, is accom- 

 plished with the greatest ease. 



The nose-piece carrying the Wen- 

 ham prism is readily removed and 

 another one substituted having no 

 diaphragm. 



The substage and mirror are at- 

 tached to bars, both of which swing 

 on a pivot around the object as a 

 centre ; they both have a silvered 

 and graduated limb, and can be 

 swung together or separately. When 

 these two bars are brought together 

 they are held by a spring stop ; an- 

 other spring stop indicates when they 

 are brought to the centre. 



The swinging substage may be 

 used for measuring the angular aper- 

 ture objectives. This method con- 

 sists in attaching a small lamp to the 

 substage and using it as a source of 

 light for observing some object upon 

 the stage of the microscope. By 

 gradually swinging the substage and 

 lamp until the image disappears, we 

 have measured all the usable angular 

 aperture of the objective. The sub- 

 stage is moved up and down upon its 

 bar by a rack and pinion, as usual. 

 Its centering arrangement is placed 

 behind the ring, and consists of a 

 rack and pinion, and a screw similar 

 to the movement of most mechanical 

 stages. It has a motion from side to 

 side of about half an inch, and a 

 quarter of an inch forward and back. 

 An arm beneath the stage holds the 

 hemispherical lens or Woodward 

 prism. This arm can be removed. 

 The lens or prism can also be placed 

 in the substage. In the latter they 

 can be centered with great ease. Be- 

 low the substage is a ring to hold the 

 polarizer. This ring swings to one 

 side, enabling the observer to examine 

 an object alternately with polarized 

 and.unpolarized light, without trouble 

 or disturbing the object. 



The stage is held by a saddle- 

 piece which is steadied by a strong 

 brace passing down from the limb. 

 It is entirely independent of the 

 swinging of the mirror and substage. 

 This saddle-piece contains a set of 

 screws with perforated heads for 

 centering the ring which supports the 

 stage. By removing these screws so 

 far back the ring can be very much 

 reduced in thickness without impair- 

 ment in strength or stiffness. The 

 stage rests upon this ring. It rotates 

 and can be centered with the greatest 

 exactness by the screws in the saddle- 

 piece. 



The stage is a revival of an idea 

 which Mr. Bulloch says was applied 

 by Spencer thirty years ago. It con- 

 sists of the ordinary stage-plate, hav- 

 ing in its centre a large square hole. 

 One side of this plate contains a wide 

 dove-tailed groove. In this groove 

 slides a bar with its surface level with 

 the top of the plate. At right angles 

 to this bar is attached another bar. 

 On this second bar slides a third bar, 

 into which it has been dove-tailed. 

 The motion of this third bar is, of 

 course, at right angles to the motion 

 of the first. A thin plate is attached 

 to the third bar and lies flat upon the 

 stage-plate. This plate is perforated 

 and holds the slide by means of a 

 spring. It will be seen that this ar- 

 rangement permits of motion of the 

 thin plate in two directions at right 

 angles to one another. Two pinions, 

 perpendicular to the stage, which 

 work one through the other, and act 

 upon racks placed at right angles, 

 effect this motion. Scales placed at 

 right angles serve the purpose of 

 finders. 



This form of stage presents several 

 advantages. It is convenient. The 

 perpendicular milled-heads of the 

 stage-motion are within easy reach of 

 the hand, and from them a change 

 can be made to the milled-heads of 

 the coarse or fine adjustment, without 

 thought or trouble. 



The motion of the stage is very 

 easy and rapid ; an animalcule can 



