98 



CONSTRUCTION OF THE MICROSCOPE. 



FiQ. 18. 



was the fundamental requirement (see Introduction, p. 61). Tlie 

 tripod foot (Fig. 18), with its pair of uprights, is of cast iron ; and 

 affords a very firm and steady basis of support. The centres of 

 suspension by which the microscope is swung between these up- 

 rights, are attached to the hol- 

 low pillar that bears all the 

 other parts. Just above them, 

 when the instrument is in a 

 vertical position, is a milled 

 head on either side, which 

 acts on a rack cut into the 

 stem that rises from the pillar, 

 and carries the body on a 

 transverse arm, thus giving 

 the "coarse" adjustment for 

 focal distance ; whilst the 

 "fine" adjustment is given by 

 another milled head (seen 

 edgeways in the figure) in the 

 transverse arm, which turns 

 a screw whose extremity acts 

 upon a lever that produces a 

 slight change in the distance 

 between the object-glass and 

 the object, by elevating or de- 

 pressing a tube that carries 

 Field's Compound Microscope. ^j-^g former, — this tubc being 



80 fitted to the lower end of the body as to slide freely within it, 

 and being pressed downwards by a spring, whilst it is raised up- 

 wards by the lever-action just named. The additional advantage 

 is gained by this arrangement (which is the one adopted with 

 some modification by most Microscope makers), namely, that if 

 the object-glass should be carelessly forced down so as to press 

 upon the object, the yielding of the spring-tube prevents any 

 serious injury to the one, and to a certain extent protects the 

 other. The stage, which is firmly attached to the pillar, is fur- 

 nished on its upper surface with a movable brass ledge, against 

 which the object rests when the stage is inclined in any degree 

 to the horizon ; this ledge should slide smoothly and easily from 

 the back to the front of the stage, hut should have at the same 

 time suflicient hold upon it to retain its position and to support 

 the object, at whatever point it may be left. At a little distance 

 beneath the stage, there is attached to it a " diaphragm plate," 

 perforated with holes of various sizes for the regulation of the 

 quantity of light admitted to transparent objects (§ 55), and also 

 aflbrding, in one of its positions, a dark background, which is 



separate stand, stage-forceps, and live-box, in a mahogany case, is only three guineas; 

 and the maker, Mr. G. Field, of Binningham, is bound by his agreement with the 

 Society of Arts to keep it always in stock, so as to supply any purchaser at once. 



