134 Globules for Microfccpts.—lmptrfeBions of the Plumb- Line. 



The tliinneft tin-foil in fmall fheets for filvering looking-glafles, is about one-thoufandth 

 of an inch thick, or near three hundred times the thicknefs of gold leaf. It ufually con- 

 tains lead. 



3. Globules for Micrcfcopts. 



AT the beginning of the prefent century, the fimple microfcope was very much ufed. 

 Among other aJvantages, it poflefles the very defirable requifites of fimplicity and cheapncfs. 

 In particular it is an inftrument not difficult to be conftrucled by fuch ingenious philofo- 

 phers as by narroiv circumflances and remote fituations are obliged to have recourfe to 

 their own (kill and ingenuity for experimental implements. The hiftory of natural 

 philofophy abounds with indanccs of eminent men who come under this defcription. 

 To thefe at lead it will be of importance to know a ready method of forming rcry biight 

 fpherules of glafs for microfcopic ufes. The ufual method has been to draw out a fine 

 thread of the foft white glafs called cryftal, and to convert the extremity of this into * 

 fpherule by melting it at the flame of a candle. But this glafs contains lead, which is 

 difpofed to become opake by partial reduftion, unlcfs the management be very carefully 

 attended to. I find that the hard glafs ufed for windows feldom fails to afford excellent 

 fpherules. This glafs is of a clear bright green colour when feen edgeuays. A thia 

 piece was cut from the edge of a pane of glafs Icfs than one-tenth of an inch broad. 

 This was held perpendicularly by the upper end, and the flame of a candle was direcled 

 upon it by the blow-pipe at the diftance of about an inch from the lower end. The glafs 

 became foft, and the lower piece defcended by its own weight to the diftance of about 

 two feet, where it remained fufpended by a thin thread of glafs about one five-hundredth 

 of an inch in diameter. A part of this thread was applied endways to the lower blue 

 part of the flame of the candle without the ufe of the blow-pipe. The cr.tremity im- 

 mediately became white-hot, and formed a globule. The glafs was then gradually and 

 regularly thruft towards the flame, but never into it, until the globule was fufficiently 

 large. A number of thefe were made, and, being afterwards examined by viewing 

 their focal images with a deep magnifier, proved very bright, perfe£V, and round. 



4. On the Plumb-Line and Spirit-Level. 



THE fpirit-level is an inftrumcnt fubftituted inflead of the plumb-line for determining 

 tlie level. It feldom happens that wire can be procured fo fine as the one-thoufandth of 

 an inch in diameter. Upon examining this with the microfcope, it fcarccly ever proves 

 truly cylindricaL It may with reafon be fuppofed that wire, from its curl upon the bobbin, 

 and its elafticity, may not, even when loaded with a confiderabie weight, form an accurate 

 right line. As the radius of a circle is equal to 206265 feconds of meafure nearly, the 

 diameter of fuch a wire would cover a fpace on the line equal to 2c6 feconds on an inftru- 

 ment of one inch radius, or 17 feconds if the radius were 12 inches. How far this 

 quantity, in the bifeflion of two dots, of near half a minute broad, may be afieiflcd by the 

 caufes of error here mentioned, mud be afcertained, in each particular inflancc, by careful 

 e.xperiments. But thefe confiderations, joined to the circumftance that the fpirit-level 

 is fomewhat more portable and eafy of application, have tended to bring this laft very de« 

 licate indrument into edimation with adronomers. 



The 



