230 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



<>J ' ilii* instrument ha.- more recently been intro- 

 duced liy the firm of ('has. Baker, of Holborn, London. It arose in 

 a suggestion I iy .Mr. Xel-on that thi.- form should be adapted to the 

 Campbell <1 i[f erential screw fone adjustment, making a good quality 

 bhird-class microscope. It should be noted that the differential 

 screw permits of slow action being obtained by means of coarse 

 tln-i'tiilx; it is therefore very strong. In the ordinary Continental 

 form of direct -acting fine-adjustment screw, if the motion is slrm\ 

 tlie thread must be fine. Hence in forms where the fine adjustment 

 is made to lift the body, the differential screw is of great value. 



Further, it proved on testing that the Campbell differential screw 

 was eijual to the most critical work, and could be used in photo- 

 micrography. As a result several additions were made, such as 

 rack and pinion focussing and rectangular movements to the sub- 

 stage and a rack-work arrangement to the draw-tube. Subse- 

 quent Iv a larger and heavier instrument was made, having a J inch 

 more of hori/.ontal height. In this model the milled head of the 

 differential screw is placed below the arm, instead of above it, which 



is an improvement for 

 ] ihoto-niicrographic pur- 

 poses, and no special 

 detriment in ordinary 

 work ; and, if required, 

 a differential-screw fine 

 adjustment can be fitted 

 to the sub-stage. A 

 rotary stage is also some- 

 times put to this instru- 

 ment, but those which 

 we have seen have not 

 given the aperture .suffi- 

 cient dimensions for 

 modern focussing. 



This instrument in its complete form, as suggested by Mr. Nelson 

 and devised by Baker, gave origin to an entirely new group of 

 microscope-, which aimed chielly ;,t supplying the student with 

 relatively inexpensive Instruments, but winch at the same time 

 should possess ,-dl the ijiialities a nd be capable of receiving all t he 

 apparatus neei I fill i;,r an eili.-ient use of the microscope. One of the 

 higher form- arising in this neu departure is the instrument shou n 

 at fiii. 177. and. \\ith the Campbell screw fitted behind the mirror 

 lor the line adjustment of the condenser, is a very attractive and 

 useful microscope. ;md may lie safely recommended to the amateur 

 and t he -t udent . 



T\\o microscopes by Ko certainly deserve the attention of the 

 student seeking a reliable in-,t rument belonging to the class we are 

 onsideriiin-. They are both known as - Ros-'s New Bacteriological 

 Microscope.' The work of this long-established firm.it is needless 

 to say, is of the very line>t <jnalit\ : and these microscopes are pro- 

 wit h all the required adjunct- for the work they specify. The 

 of horse-hoe form; the fine adjustment is sensitive and firm. 



FIG. 183. Ceutriiu nose piece u-t-<l as Mill-stage 

 (1881). 



