THE COMPOUND MICROSCOPE. 71 



those in a three-legged stool. Some makers even use a 

 heavy circular foot instead of a tripod, but this, although 

 steady when the instrument is upright, is not so when it is 

 inclined. From a foot of one of the above forms a stout pillar 

 rises, having at its upper part a cradle-joint, to which both 

 compound body and stage are firmly attached, so that when 

 the joint is used, both these parts move together. Mr. George 

 Jackson, having anticipated some difficulty in making a good 

 cradle-joint, was induced to use two pillars instead of one, by 

 which a greater degree of steadiness was obtained ; his com- 

 pound body and stage were connected to both pillars by 

 trunnions, on which they were made to turn. Mr. Jackson 

 also mounted his compoimd body on a grooved beU-metal 

 arm, — a plan somewhat similar to that adopted by Mr. Ross, 

 in one of his early microscopes,* and which wiU be more fuUy 

 described in the instruments of Messrs. Smith and Beck, who 

 have adopted it. Mr. Eoss uses the tripod foot, and two flat 

 supports, by which the same end is accompKshed as by 

 pillars; but the supports, he considers, are much more free 

 from vibration. In some of the recently constructed portable 

 instruments, the stage is mounted on a strong pivot, on 

 which it can be turned in the same direction as the compound 

 body, for convenience of package. The smallest instru- 

 ments of Messrs. Powell and Koss are constructed on this 

 principle. 



The optical part of the compound microscope consists of the 

 object-glasses, the eye-pieces, and the mirror. The object- 

 glasses supplied with the best instruments are generally either 

 five, six, or seven in number, and vary in their magnifying 

 power from 20 to 2,500 diameters ; they are called two-inch, 

 one -inch, half-inch, one-quarter, one-eighth, one-twelfth, and 

 one-sixteenth; but it must be understood that these names 

 are not derived from the distance the bottom-glass of each 

 combination is from the object, but from a fact found in 

 practice, that a thin single lens, to magnify the same number 

 of diameters as any of the preceding achromatic combinations, 



* Art., Microscope, Penny Cyclopcedia. 



