10 



THE AMERICAN MONTHLY 



[January, 



era, known as Walker's pocket-cam- 

 era, above the eye-piece, \^dth the 

 microscope vertical, and with this 

 photographs objects of all kinds, 

 even in liquids. In a recent letter 

 he wrote as follows : — ' I have had 

 an adapter made which slips over 

 the eye-piece and accurately fits the 

 tube containing the landscape lens, 

 thus giving rigidity to the whole ap- 

 paratus. By keeping the landscape 

 lens in the tube, one loses light, it is 

 true, but no correction is necessary 

 for actinic focus. I have other and 

 larger cameras, but generally use the 

 pocket-camera when the object will 

 permit.' 



Dr. Sternberg's apparatus differs 

 from all others in that his camera is 

 suspended upside down from a hea^•y 

 wooden beam at such a height that 

 focussing is done while standing 

 erect. This is a plan to be highly 

 recommended ; and we are in- 

 clined to think it would be better to 

 suspend it still higher, so that one 

 can walk under the apparatus, 

 mounting on a small platform to fo- 

 cus and manipulate. It would sa^-e 

 considerable floor-space, and would 

 not be so inconvenient as might be 

 supposed at first thought. 



We must now proceed to consider 

 the subject of illumination, and the 

 apparatus for that purpose. 

 [To be co>ifiiiued.'\ 

 O 



Bulloch's Lithologlcal Microscope 

 Stand. 



The general construction of this 

 stand, which has been mentioned be- 

 fore in these columns, is shown in the 

 illustration (fig. 6), It is similar to 

 the professional stand, except in the 

 following details : — There are two 

 stages, one with a plain, sliding object- 

 carrier. Each stage is graduated to 

 fifteen minutes of arc, reading by a 

 vernier to twenty seconds, and can 

 either be revolved by hand or by a 

 tangent-screw, as shown in the illus- 

 tration, which also acts as a slow^ mo- 

 tion. The worm cut on the periphery 

 of the stage has 360 teeth equal to 



single degrees, and the tangent-screw 

 head is divided into sixty parts, so 

 that each division reads to one min- 

 ute. The tangent-s cr evs^ can be 

 thrown in or out of connection as re- 

 quired. This arrangement is com- 

 mon to both stages. The second 

 stage, as shown in the illustration, is 

 furnished with a sliding object-car- 

 rier, and with micrometer screw 

 movements in two directions for the 

 direct movement of objects without 

 reference to magnification. The 

 screw-threads are one-half millime- 

 tre, the head being graduated to 350, 

 so that each division reads to two 

 microns (.003 mm.) which may be 

 again subdivided by a vernier into 

 tenths. At the side of the limb there 

 is a scale reading to half millimetres, 

 and the slow motion screw-head is 

 graduated to 300 divisions, reading 

 to I micron. The polarizing prism 

 below, fitting in the sub-stage, has a 

 graduated circle of degrees and a 

 spring catch at each 90°. The analyz- 

 ing prism at the lower end of the 

 body-tube has a revolving movement 

 by a lever of 90°, and can be removed 

 bv a slide similar to that of Wenham's 

 binocular prism. There is also at the 

 lower end of the tube a Klein quartz 

 plate, and a centring nose-piece. There 

 is a gonometer eye-piece with crossed 

 spider lines, a Nichol prism, and a 

 calc-spar plate. The fitting is made 

 adjustable, for if the calc-spar is not 

 cut in the proper direction, the cross 

 cannot be placed in the centre of the 

 field without slightly tilting the crys- 

 tal. To change from polarized to 

 ordinary illumination the prism be- 

 lo^v the stage can be turned aside, 

 leaving the wide-angle condenser in 

 position ; or the whole sub-stage can 

 be turned aside, which inovement is 

 supplementary to swinging on an axis 

 with the object on the stage as a 

 centre. When the condenser is not 

 required there is a supplementary sub- 

 stage for the lower prism, so that the 

 prism can be used close to the object, 

 and no light admitted except that 

 which has passed through the prism. 



