INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



869 



readily revealed, especially sucli diatoms as have rectangular strisB 

 or markings 



With diatoms easily resolvable, and only requiring one pencil of 

 light to sliow the markings, the second arc-piece with its illuminating 

 apparatus can be turned away from the stage as shown in Fig. 83. 

 The figure also shows how ojiaquc objects may be illuminated, viz. 

 by moving the condenser of the first arc-piece above the stage of the 



Fig. 83. 



Microscope, when a pencil of light can bo projected on to the object 

 more perpendicularly than with the buH's-cyc condenser, thus pre- 

 venting shadows in coarse or deep objects which often produce dis- 

 tortion and false appearances. When the apparatus is used for opaque 

 objects with a lower power than the 1-inch objective, the achromatic 

 combination can bo removed and the light directed from the prism, 

 which can be made to give convergent rays sufficient for use with a 

 4-inch objective. 



A great advantage is presented in this arrangement, viz. that a 

 more obli({uo angle of light can bo obtained with it tlian by other 

 swinging stages, in consequence of the optical combination (with all its 

 fittings included) being less than ,V the diameter of the establislied 

 size of substage used with the Zcntmaycr, for instance, thus enabling 

 the apparatus to bo moved further up to the under stirfuce of tho 

 stage than if its mountings were larger. A still further improvement 

 claimed for it is that tlie whcde of the apparatus and its belongings 

 can be easily detached from tho Microscope, and an ordinary substago 

 slid into tlie same fitting for tlio purpose of receiving pcdarizing 

 apparatus, paraboloid, spot lens, &c. As its iitting is not adapted to 

 or coimected with tho stage, tho firmness and stability of this very 

 important portion of tho instrument are not thereby imi)airod." 



