230 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



Another form of tJds instrument has more recently been intro- 

 duced by the firm of Chas. Baker, of Holborn, London. It arose in 

 a suggestion by Mr. Nelson that this form should be adapted to the 

 Campbell differential screw fine adjustment, making a good quality 

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

 screw permits of slow action being obtained by means of coarse 

 threads ; it is therefore very strong. In the ordinary Continental 

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

 the 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 equal 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- 

 quently a larger and heavier instrument was made, having a J inch 

 more of horizontal 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 

 photo-micrographic 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 

 FIG. 183. Centring nose-piece used as sub-stage 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 

 microscopes, which aimed chiefly at supplying the student with 

 relatively inexpensive instruments, but which at the same time 

 should possess all the qualities and be capable of receiving all the 

 apparatus needful for an efficient use of the microscope. One of the 

 higher forms arising in this new departure is the instrument shown 

 at fig. 177, and, with the Campbell screw fitted behind the mirror 

 for the fine adjustment of the condenser, is a very attractive and 

 useful microscope, and may be safely recommended to the amateur 

 and the student. 



Two microscopes by Ross certainly deserve the attention of the 

 student seeking a reliable instrument belonging to the class we are 

 considering. They are both known as ' Ross's New Bacteriological 

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

 to say, is of the very finest quality ; and these microscopes are pro- 

 vided with all the required adjuncts for the work they specify. The 

 stage is of horseshoe form ; the fine adjustment is sensitive and firm. 



