OF OPTICS. 249 



fected by passing through concave glasses, and also by LECT, 

 falling upon concave mirrors. v^v^v 



smallest sort are so short, that it requires considerable attention to employ 

 them well. F. Di Torre, of Naples, in 1765, sent several glass globules 

 to the Royal Society. The largest of them was only two Paris points in 

 diameter, and is said to magnify the diameter of an object 640 times ; 

 another was the size of one Paris point, magnifying the diameter 1280 

 times ; and the smallest no more than one half of a Paris point, or the 

 144th part of an inch in diameter, and is said to magnify the diameter of 

 an object 2560 times ; and consequently the square of that diameter 

 6, 553,600 times. Globules so exceedingly minute as these, were at one 

 time highly prized, but spherule microscopes are not now made so small, 

 to avoid straining the eyes. The third, or smallest globule above-men- 

 tioned, could only be the 576th part of an inch distant from the object, 

 because the focus of a glass globe is at the distance of one-fourth of its 

 diameter ; it is obvious therefore that it could admit very little light, and 

 could not be used without pain and difficulty even by practised observers, 

 Of the various methods which have been recommended for making glass 

 spherules, the following by Nicholson is perhaps the best It is observed 

 by this valuable practical writer, that the usual method has been to draw 

 rut a fine thread of the soft white glass called crystal, and to convert the 

 extremity of this into a spherule by melting it at the flame of a candle. 

 But this glass contains lead, which is disposed to become opaqne by par- 

 tial reduction, unless the management be very carefully attended to. He 

 found that the hard glass used for windows seldom fails to afford excel- 

 lent spherules. This glass is of a clear bright green when seen edgeways. 

 A thin piece, less than one-tenth of an inch broad, was cut from the edge 

 of a pane of glass. This was held perpendicularly by the upper end, 

 and the flame of a candle was directed upon it by the blow-pipe, at the 

 distance cf about an inch from the lower end. The glass became soft, 

 and the lower piece descended by its own weight to the distance of about 

 two feet, where it remained suspended by a thin thread of glass, about 

 3^th of an inch in diameter. A part of this thread was applied endways 

 to the lower blue flame of the candle, without the use of the blow-pipe. 

 The extremity immediately became white, and formed a globule. The 

 glass was then gradually and regularly thrust towards the flame, but 

 never into it, until the globule was sufficiently large. A number of these 

 were made, and being afterwards examined by viewing their focal images 

 w ith a deep magnifier, proved very bright, round, and perfect. 



mles are mounted for use by placing them between two very thin 

 plates of brass, each containing a small hole rather less than themselves. 

 It any imperfection in the globule is discoverable, it is placed on one 

 side, so that it may be covered by the plates. The objects may be placed 

 on the point of a needle, the direction of which should be at right angles 

 to the axis of the eye, to prevent accidents. 



