90 MICROSCOPIC ILLUSTRATIONS. 



a quarter round to the left, which brings the milled 

 head #, upwards, and the arm, n, a, horizontal, where 

 the stem, c, is fixed by the pinching screw, z. In this 

 position a candle may be placed close behind the stage in 

 the axis of the instrument. This will be found of great 

 advantage in verification. Again, if the stage be re- 

 moved, objects in glass vessels, which are too large to be 

 held in the microscope, may be examined without re- 

 moving them for the purpose; as also minerals, blocks 

 of fossil woods, &c. 



The universal or ball-and-socket joint being strongly 

 recommended in Chap. VII., it may with propriety be 

 asked, why I have introduced a different one into my own 

 instrument. The reason is, that while all the requisite 

 motions are obtained by means of the joint I have 

 adopted, it allows, at the same time, the stem of the 

 microscope to be over the centre of support, by which 

 it has no tendency to fall, when the pinching screw is 

 loosened. In the universal joint, there is always a 

 tendency to drop (except when it is on its side), and 

 a strain upon it. In practice, therefore, these are very 

 liable to get out of order, where the weight of the in- 

 strument is considerable. When formed on a small 

 scale, this objection vanishes; but when of the size 

 adopted by Dr. Goring, until a joint can be constructed 

 similar to that in the human arm, with a ligament in 

 the centre, and sufficient friction to support the weight 

 in any position, no person, I conceive, will give them the 

 preference : at least, so it has been 5 for where one 



