598 SUMMARY OF CURRENT RESEARCHES RELATING TO 



marked C. In reference to this Pritchard says (" Microscopic Illustra- 

 tions ") : " The above sets may be divided into three classes — the 

 shallow, medium, and deep ; which, like the eye-pieces, I shall particu- 

 larise by the letters A, B, C." From the above marks it may be 

 concluded that -^ in. and all above are classed as C, from -^ in. to | in. 

 as B, and ^ in. and below as A. The 1 in., 1^ in. (?), and 2 in. have 

 lieberkuhns. 



According to a table in "Microscopic Illustrations," the working 

 distance of the objectives is : for the 2 in. = 2^ in. ; 1 in. = 1 j- in. ; 

 4- in. = | in. ; £ in. = -£$ in. 



There are five oculars, two having micrometers, one being a simple 

 scale engraved on a thin slip of mother-o'-pearl. There is an arrange- 

 ment for focusing the eye-lens on the micrometer scale. The other 

 micrometer eye-piece is ruled in squares for use in making drawings. 



Among the apparatus is a camera-lucida, a dark well, some live 

 boxes (one with a scale engraved on the bottom of the cell), and a 

 candlestick or lamp-holder, shade, and brackets. Of the candle-holder 

 Pritchard seems to have been rather proud. 



There is an additional body, about 8 in. long and about 1 T 7 ^ in. 

 internal diameter, but there is neither fine-adjustment nor any means of 

 fixing the body to the stand, though at one time there was doubtless 

 either another arm, or an adapter, to enable it to be screwed into the 

 existing arm. 



There are three oculars belonging to this body, one having a micro- 

 meter ruled in squares, with a collar arrangement for focusing it to the 

 eye-lens, another ocular has a double eye-lens, the outer one being a 

 meniscus. 



There is also an adapter for applying the analyser above the 

 objective. 



The figure shows the stand with the candlestick and shade in 

 position. To the left at the back is the " spring-phial- holder," and just 

 in front of the foot is the camera-lucida ; near the middle in the fore- 

 ground is a dark-well and the polarising apparatus— polariser and 

 analyser screwed together; near the point of the right foot is the 

 micrometer ocular belonging to the large body ; the other items are a 

 selection of the objectives and oculars. 



Mr. Nelson is of opinion that the instrument was undoubtedly made 

 by Hugh Powell for Pritchard, and he calls attention to the spring 

 joints to the mirror. This device for spring joints was used by Powell 

 as early as 1831, and is still used by Powell and Lealand. 



Zeiss' Martens Stand. — The special Microscope (fig. 65) for metal- 

 lurgical work, constructed by Messrs. Carl Zeiss, Jena, in accordance 

 with the suggestions of Professor Martens and Engineer E. Heyn, of 

 the Konigliche technische Versuchsanstalt, Berlin, consists of a solid 

 base so arranged that it can be fastened to a sole-plate on an optical 

 bench. The stand is chiefly constructed for photomicrography, and is 

 not inclinable. The mechanical stage is attached to a movable dove- 

 tailed slide provided with a rack-and-pinion and a micrometer screw 

 for fine-adjustment, by means of which the objects can be focused 



