Theory of Illuminating Apparatus. By Dr. H. E. Fripp. 529 



thence to the mirror 3'4 inches. In this position, therefore, a parallel beam of 

 incident light would focus below the object, but a slightly divergent incident 

 light-cone coming from a near light source, would fall with conjugate focus on the 

 object. Between these extreme positions the vertical movement of the condensing 

 lens Q inch) would afford various intermediate incidence of light rays. The 

 absence of tiie diapliragm and the fixed distance of mirror render the mobility of 

 condenser essential for regulation of light and disposition of conjugate focus. 



The mechanical arrangements of this simple Microscope are specially worthy 

 of notice, as indicating transitional phases in construction. The tube form of 

 body and fixed distance of mirror are retained as in Wollaston's instrument — 

 obviously the prototype of the " petit tambour " model so common in Swiss and 

 French instruments. The absence of diaphragm no less clearly marks the fact 

 that its function as a regulator of tlie magnitude of tlie ilhmiinating pencil was 

 not recognized, or at least not acted upon, as an optical principle to be taken 

 advantage of in construction of tlie illuminating apparatus. On the other hand, 

 a great advance in the general mechanical arrangements beyond those of Wol- 

 laston's instrument, is noticeable in the addition of a small traversing stage, with 

 revolving object carrier, and of a " fine movement " screw to the stem and bar, 

 carrying the simple or doublet lenses. Thus we find in Dollond's developments of 

 the Wollaston instrument the same mechanical aids which characterize the 

 modern stage movements of the compound Microscope with the exception of those 

 introduced in the more complete arrangement of the sub-stage and movable 

 mirror which followed the later practice of oblique illumination and the applica- 

 tion of various accessory apparatus. 



The main interest attaching to this instrument lies, however, in the attempt 

 to develop the means of Illumination upon scientific principle. 



