358 BB;NJ. PIKK'S, JR., DESCRIPTIVE CATALOGUE. 



brought to a focus or not, as required upon any given 

 object on the stage. The compound body is a tube of brass 

 about seven inches long, and one and a quarter inches 

 diameter. It contains the eye-piece at the upper end, and 

 the object-glasses at the other, screwed to an arm having 

 a square bar which is fitted to move by tooth and pinion 

 motion within the round brass column ; to its upper end 

 the eye-pieces are adapted, to its lower the object-glasses. 

 As the latter are of different magnifying powers, and also 

 the objects to be examined rarely of the same thickness, 

 it is required that there should be a focal adjustment; 

 this is effected by a tooth and pinion arrangement in con- 

 nection with the square bar sliding within the round col- 

 umn E, and motion given by turning the milled head, D, 

 by which the whole tube carrying the eye-piece and 

 object-glasses is made to approach or recede from the 

 object. The eye-piece consists of two plano-convex 

 lenses ; the smaller one is called the eye-glass, and the 

 larger the field-glass ; they are set in brass cells, and 

 mounted in a tube whose length is about equal to half of 

 the sum of their focus. Between the lenses is placed a 

 stop, or diaphragm, for diminishing the spherical abberra- 

 tion. The inside of these tubes, and also of all other 

 parts through which the light passes, is blackened, so 

 that no other rays than those from the mirror should in- 

 terfere with the illumination. The achromatic object- 

 glasses are three in number, and screw together, being 

 so arranged that one, two, or three may be used according 

 to the magnifying power required. There is also a double 

 convex lens in a cell, useful where a low power is required. 

 The condenser, H, or illuminating lens for opaque objects, 

 is a plano-convex lens of short focus set in a brass frame, 

 and supported by one of its arms in a socket attached to 

 the stage-plate ; the arms are jointed, and the lens may 

 be set in any position required to receive the light and 

 condense it on the object to be examined. The steel for- 

 ceps, I, for holding small objects, are formed at the end of 

 a small wire, and made to slide in a small tube having a pin 

 projecting to fix it in a hole in the stage ; the forceps are 

 opened by pressing small studs between the thumb and 

 finger. The dissecting-knife, M, and point, N, are con- 

 venient for separating parts of objects, and the brass for- 

 ceps, 0, for taking up small objects. The fluid-box, R, 



