1068 



SOMMAEY OP CUBRENT RESEARCHES RELATING TO 



Table of Colour-corrections-* — Mr. J. W. Queen gives the follow- 

 ing table of the colour-corrections of objectives : — 



Under-correction 



Slightly under — but a large number of) 



the finest lenses have this colour . . / 

 Nearly colourless — shows the secondary"! 



spectrum / 



Over-correction 



WilhiD Focus. 



Brick red 



Claret 



Lilac 

 Blue 



Without Focus. 



Greenish blue 

 Light green 



Paler green 

 Yellow 



Joly's Meldometer.t — The apparatus which Mr. J. Joly calls by 

 this name (/x,eA.8a>, to melt) consists of an adjunct to the mineralogical 

 Microscope, whereby the melting-points of minerals may be compared 

 or approximately determined, and their behaviour watched at high 

 temperatures, either alone or in the presence of reagents (figs. 240-1). 



As now used, it consists of a narrow ribbon of platinum (2 mm, 

 wide) arranged to traverse the field of the Microscope. The ribbon, 

 clamped in two brass clamps so as to be readily renewable, passes 

 bridgewise over a little scooped-out hollow in a disc of ebony (4 cm. 



Fig. 240, 



diam.). The clamps also take wires from a battery (3 Grove's cells), 

 and an adjustable resistance being placed in circuit, the strip can be 

 thus raised in temperature up to the melting-point of platinum. 



The disc being placed on the stage of the Microscope, the platinum 

 strip is brought into the field of a 1 in. objective, protected by a glass 

 slip from the radiant heat. The observer is sheltered from the intense 

 light at high temperatures by a wedge of tinted glass, which further 

 can be used in photometrically estimating the temperature by using 



* Queen's Micr. Bull., ii. (1885) p. 38. t Nature, xxxiii. (1885) pp. 15-16. 



