103 The Microscope. 



great advantage to use a prism of very great dispersion ; for if 

 this be done the bands are rendered so wide and nebulous as to 

 be seen only with great difficulty. 



This instrument is hardly suited to the study of those 

 bright lines which are given by the glowing vapors of the va- 

 rious metals, etc., as its dispersive power is not sufficient to sep- 

 arate them enough to be accurately mapped. Still many of 

 them can be determined providing the lines are few and quite 

 far apart. To determine the pressure of a well known element 

 in this way, it is not always necessary to obtain the entire num. 

 ber of lines which its glowing vapor will give ; but simply select 

 the characteristic, or a line as is shown in No. 11, of the plate 

 which represents the brilliant red a line of lithium vapor as 

 given by an alcohol flame. The other line of the spectrum of 

 this metal is fainter, of an orange color, and in position a" little 

 to the left of D. However, the determination of the exact 

 position of the red line is sufficient in determining the presence 

 of lithium. 



For the determination of the presence of such metals as 

 only volatilize by exceedingly high temperature, electricity 

 may be had recourse to, by way of a spark from the Ruhmkorf 

 induction coil, or the oxy hydrogen blow-pipe may be used. 



Prof. Sorby has recently devised a form of microspectro- 

 scope which is used as an objective— that is it screws to the nose 

 of the microscope. This may be used with the binocular, but is 

 not as well suited for accurate work as the form here described. 



Prof. W. A. Rogers, of Cambridge, has presented us with 



an exceedingly rare and valuable slide, of " glass threads." 



The ruling diamond has thrown up an even, unbroken windrow 



of glass, on the side of the groove it cut. These windrows are 



made in rulings, only under certain rare conditions, and Prof. 



Rogers has only obtained a very few of these slides in all the 



years of his work. 



-*•*- 



In the article by Dr. Stokes the sentence '• with an amplifi- 

 cation of 350 diameters or less," immediately under the illus- 

 trations, has no reference to the figures and should be placed at 

 the top of the key, forming the first line. 



