no 



APPENDIX. 



Fig. 431. 



support, to go with his general instrument, and so constructed 

 it, that it could be inverted so as to have the stage above, and 

 the objective' below the object; but every one who has used it 

 knows how awkward it is for manipulation, although exceed- 

 ingly ingenious. Dr. Smith, impressed with these difficulties, 

 was led to the construction of the instrument, which, in this 

 country, bears his name. 



It was important for the arrangement in question, so to have 

 the relative position of the stage and eye-piece, that the eye, 

 while on a level with the latter, could readily see the former, and 

 guide the required manipulation. 



The most important part of the instrument is a four-sided prism, 

 Fig. 430, with the angles ab, he, 

 cd, and da, respectively 55°, 

 107J°, 521°, 145°^ the angles 

 being of such dimensions that 

 a ray of light passing into the 

 prism in the direction _E'a, and 

 perpendicular to the upper sur- 

 face of the prism, after undergo- 

 ing total reflection from the in- 

 ner surfaces (on both of which 

 the light strikes at an angle 

 much less than forty-five de- 

 grees), will pass out perpendi- 

 cular to the surface connected 

 with the body of the instru- 

 ment. 



It will be readily seen how 

 a ray of light, entering the 

 object-glass, descends into the prism, and passes out of it up- 

 wards, through the eye-glass, the tube of which is inclined to the 

 perpendicular 35°. The other parts of the instrument will be 

 understood by looking at the iigure. (Fig. 431.) The illumina- 

 tion of the object is effected by a prism, instead of a mirror. 



In examining an object with this microscope, the object is 

 arranged in the ordinary way ; when liquid, it is placed in a 

 watch-glass, or such glass cells as are convenient to use. In 

 employing reagents, they can be added and watched immediately, 

 for it is readily seen how the eye guides the manipulation on the 

 stage and looks into the instrument almost at one and the same 

 time. 



In observing with high powers, as the object-glass is beneath 

 the glass supporting the object, and as the glass is usually of a 

 certain thickness, the method of observation must be changed. 

 For all powers resorted to in chemical examination this difficulty 

 never occurs, and in using high powers it is easily obviated. 

 "When the object is already mounted and dry, the thin glass can 

 be readily turned downwards ; but where it is moist, as for in- 



