1 86 THE HISTOKY AND DEVELOPMENT OF THE MICROSCOPE 



for this end which will imperil the absolute firmness of the sub-stage, 

 or else more will be lost than can be gained. The need of such a 

 device for the most delicate and critical microscopical work is shown 

 plainly by the fact that during the past few years several ingenious 

 and practical devices have been used, nearly every principal Eng- 

 lish maker employing a method of his own. The first arrangement 

 was made in Powell and Lealand's sub-stage and is shown in fig. 148. 

 The nature of this device, which was suggested by Mr. Nelson, will 

 be readily understood. It does not interfere with the general 



FIG. 147A. Attachable stage with diaphragm in the plane of the stage. Top 

 view and cross section showing construction of stage and attachment of iris dia- 

 phragm. 



mechanical arrangements of the sub-stage ; it will be seen that the 

 milled head A controls a screw spindle terminating in a steel cone B. 

 On rotating A, B turns, and with a very slow motion forces up (or 

 releases as the case may be) a pin C, inserted in the base plate E of 

 the sub -stage. The motion of C carries with it the condenser. At 

 right angles to and forming part of E at the back an inner sliding 

 plate works against a spring at the upper end between bearings F at 

 each side, which are fixed upon the usual racked slide D of the sub- 



