184 



KNOWLEDGE. 



May, 1913. 



Plan 



or source of light ; between these must be fixed the 

 stage for carrying the object under examination. 



The Stand of the microscope consists generally 

 of two parts, the foot or base which supports the 

 instrument, and the limb which is connected to the 

 upper part of the foot through an inclining joint 

 permitting any angle of inclination from the vertical 

 to horizontal. 



Much discussion has taken place concerning the 

 relative merits of the two styles of base generally in 

 use, namely, the tripod or so-called English style, 

 and the horseshoe or so-called Continental style ; 

 the former, no doubt, gives greater stability to the 

 instrument in various positions, and the latter provides 

 more room for the manipulation of the 

 substage fittings, especially if these are 

 required to be swung out from the optical 

 axis. A little consideration will shew how 

 these advantages can be combined in a 

 suitable design. 



The Limb should be one continuous 

 piece to which all other parts can be 

 attached either rigidly or through slides 

 which allow for the requisite vertical move- 

 ment. The limb should provide ample room 

 between the top slides carrying the body 

 tube and the stage, so that a low power 

 objective (with a working distance of, say, 

 three inches) can be used when a nosepiece, 

 or objective changer, has been added to 

 the tube. 



At the present time other appliances are 

 supplied for attachment to the body, such 

 as a slot for quartz wedge, a vertical 

 illuminator, and so on, for which the space 

 provided is generally quite inadequate ; 

 some modern objectives also are of much 

 greater length than those formerly con- 

 structed. 



In my opinion the construction 

 should allow sufficient room for the 

 use of any accessories necessary 

 with a low power objective when a 

 mechanical stage has been added 

 to the stage proper ; if these acces- 

 sories are not used, and an excep- 

 tionally short objective with small 

 working distance could not be racked 

 down far enough for focusing, a 

 matching piece could be used on the 

 body tube. 



The space between the limb and 

 the optical centre could be made 

 with advantage greater than usual ; 

 it should be such as to allow for 

 a large- sized stage, say five inches 

 in diameter, to be completely rotated 

 with a mechanical stage attached, 

 and to allow the limb to be used 

 as a handle for lifting the instru- 

 ment without any strain or derange- 

 ment of the various adjustments. 



The stage is preferably carried on 

 a stage bracket ; this construction 

 allows for a wide choice of the style 

 of stage and also provides for a 

 slight adjustment to insure the stage 

 being exactly at right angles to the 

 optical axis. 



Another point which has been 

 much discussed is the construction 

 of the fine adjustment ; in the first 

 place, the coarse adjustment should 

 be made and finished so that it 

 works as smoothly as possible with- 

 out any sign of backlash in the 



Figure 179. 



Section through 

 body. 



Figure 180. 

 A>ith stage removed. 



I.I.I 



J 



Figure 181. 



Side elevation. Key to Figures 179, 180 and 181 



A. Stand ; B. Limb ; C. Stage; D. Body ; E. Sub 



stage; F. Tail-piece; G. Fine adjustment. 



movement or looseness in the slide ; if this is done 

 the fine adjustment need only be used for high powers 

 and need only have a small amount of vertical move- 

 ment (say one-sixteenth inch). 



The fine adjustment mechanism should be as 

 compact as possible and should act directly upon 

 the slide carrying the coarse adjustment ; there is no 

 doubt that mechanically the method involving leverage, 

 by which a greater movement is provided and less 

 weight comes upon the fine adjustment screw, is 

 preferable and more sensitive than that in which the 

 whole weight of the body is borne directly on this 

 screw, and it is difficult to see why the latter method 

 should have ever been adopted. 



In most models the under-stage fittings 

 consist of a general patchwork attached 

 indiscriminately to the stage or limb. 

 They should be connected directly to the 

 same limb which carries the stage and 

 the body tube which then forms a con- 

 tinuous support for the optical parts above 

 and below the stage. 



Sufficient room should be provided below 

 the stage for a modern condenser, with a 

 stop carrier and iris diaphragm, below 

 which can be placed a large sized polar- 

 ising prism, sufficient room also for the 

 requisite vertical movement for focusing 

 and dismounting of the various parts con- 

 veniently without interfering with the 

 mirror which is also attached to the rim 

 through the tailpiece. It is also advisable 

 that the substage fittings can be swung out 

 from the optical centre. 



The line drawings (see Figures 179- 

 181) shew a simple design which 

 complies with the requirements noted 

 above. 



The photograph (see Figure 182) 

 illustrates a more complete model 

 which was entirely designed and 

 constructed by the writer. The chief 

 differences from the design shewn in 

 Figure 181 are that the stand is 

 somewhat higher, the stage has a 

 vertical movement of one inch and 

 that a separate holder is provided 

 for a polariser below the condenser. 

 A short description of this instru- 

 ment is as follows: — 



The stand consists of two parts — 

 the base and the upright. The base 

 is supported on three feet shod 

 with cork, and having a spread of 

 seven inches by eight inches. The 

 upright part has two sides one inch 

 apart, between which the limb is 

 suspended on a pin at the top, and 

 can be inclined at any angle from 

 the vertical to the horizontal, a 

 second pin at the back supporting the 

 limb in the latter position. The 

 upright and horizontal parts of the 

 stand are connected by screws, and 

 can be taken apart when desired 

 to pack the instrument into a small 

 bulk. The limb is cast in one piece, 

 provided at the top with a dovetail 

 slide to carry the body, and at the 

 bottom with a slide to carry the 

 stage bracket and substage fittings. 

 The central portion of the limb is of 

 I section, to secure rigidity without 

 undue weight. The fine adjustment 

 is located at the top end of the limb 

 in a hollow recess ; it is actuated by a 



Inches . 



