ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1041 



smoothness and steadiness of focusing, and can be used alone for 

 focusing all powers up to 1/6 in., while the fine-adjustment is 

 claimed to be of the " greatest possible delicacy and stability, the 

 most perfect arrangement yet applied to the Jackson model form of 

 Microscope." 



The substage (figs. 202, 203, and 204) is hinged so as to give 

 increased facility for inserting and removing illuminating accessories. 

 It can also be entirely removed by a lateral slide when required. 



The mirror-stem in the Premier and Student's Microscopes (figs. 

 203, 201, and 205) is pivoted from the attachment to the limb, so 

 that it can be brought up level with or above the stage in a more 

 convenient manner for oblique illumination than with the crank arm. 



Cutter's Cam Fine Adjustment* — Dr. E. Cutter has for years 

 " sought to simplify the mechanism of the compound Microscope so 

 that a really good instrument might be had for less money — not for 

 the sake of doing away with the magnificent mechanisms now extant, 

 but for paving the way for them by making Microscopes as plentiful 

 and popular as pianos and organs." This contribution is one effort 

 in this direction. 



" Two ideas are involved : (1) To have a cam or cams at the distal 

 end of the stage, which is a steel or brass plate fastened to the proxi- 

 mal end of the bed-plate of the stage ; (2) To have four cams, one 

 at each angle of the quadrangular stage, which is drawn down to 

 the bed-plate by springs beneath, suggested by the late Dr. Elsberg. 



The advantages of the cam fine-adjustment are : — 



1. Simplicity, as compared with the screw fine-adjustment. A 

 screw adjustment is a double-faced, projecting, spiral, inclined plane 

 wound on a shaft. This plane runs on another re-entrant double 

 spiral inclined groove winding around the inside periphery of a hole 

 or shaft, usually fixed. For use on the stage the screw adjustment 

 must not wabble, yet it must move readily and have no loss of 

 motion upwards or downwards, inwards or outwards. Experience 

 has taught that it takes a skilled mechanic to make a good fine screw 

 adjustment. For ordinary screw threads the requirement is to bind 

 in one direction, but not in the other direction. Such screw threads 

 ill answer for moving backwards and forwards with the accurate 

 delicacy of such an instrument of precision as the compound Micro- 

 scope. It is expensive to make a fine screw adjustment, and there are 

 few workmen that can make them. 



On the other hand the cam adjustment is easily made by centering 

 a metallic disc, outside of the true centre, on an axis of steel wire. It is 

 simple to mount. Even an uuskilled artisan can make and mount it. 



2. Cheapness. 



3. Effectiveness. No mechanical motion is so sure and effective. A 

 short lever attached to the axis of the cam gives the means of apply- 

 ing required motions with ease and certainty. The amount of 

 motion can be regulated exactly. It is rapid and sensitive. 



4. Not liable to get out of order, as the spring or springs holding the 



* The Microscope, vi. (1886) pp. 101-4 (1 fig.). 

 Ser. 2.— Vol. VI. 3 Y 



