10 



PRACTICAL TREATISE ON 



Fig 6. 



diameter, to contain the water, which can be dropped into it 

 by a pin or large needle, and there forms a double convex lens 

 of water : c d e is another piece of brass, 

 well hammered, so as to be springy, and 

 called the object supporter; it is attached 

 to the plate a b, by the screw e; it has a 

 point for opaque objects Atf, and a hole for 

 fluids at c, both of which can be brought 

 opposite to the lens a, and can be made to 

 approach or recede from the lens by turning 

 the screw g in the round plate. This screw 

 is attached to the object supporter cde, and 

 passes throught it to the plate a b, against 

 which it works. The supporter, being made 

 springy, obeys readily the turns of the screwy. 

 Mr. Stephen Gray was also the inventor of a simple reflecting 

 microscope, represented by fig, 7. A represents a brass ring, 

 one-thirtieth of an inch thick, whose inner 

 diameter is about two-fifths of an inch. 

 Having dissolved a globule of quicksilver in 

 one part nitric acid and ten parts water, he 

 rubbed with it the inner surface of the 

 ring, which became silvered : having wiped 

 it dry, he put a drop of quicksilver within 

 it, this, when pressed with the finger, 

 adhered to the ring, and formed a convex 

 speculum. When the ring was taken up 

 ^^ I I — ^^^ carefully and laid on the margin of the 

 h- III,,! J, ^ cylinder B, the mercury sank down and 

 'W^^^^^^Piiii/ formed a concave reflecting speculum. 

 The cylinder B is supported by a pillar, 

 attached to the foot D; CC, F, G, repre- 

 sents a stage, which is capable of being 

 raised or depressed by the screw on the 

 pillar. The object is placed on the ring G, 

 and is adjusted to the focus of the speculum 

 by the abovementioned screw. 

 This ingenious gentleman, also in May, 1697, suggested the 



Fig. 7. 



