r 



t 



(,f S(tpphire and Diamond. 331 



lenses all substances that have a high refractive power, and no 

 double refraction. Substances of this kind which are suitable 

 for such a purpose, are 



Index of Refraction. 

 Realgar, - 2.549 



Blende, - 2.260 



Glass of Antimony, 2.216 for red rays. 



Glass of antimony will no doubt take a good polish. Blende 

 will probably do the same; but^ealgar is perhaps too soft. Real- 

 gar is capable, however, of being melted, and we have no doubt 

 that small transparent lenses of it could be moulded between 

 small polished concave surfaces. We formed in this way a 

 prism which enabled us to obtain a tolerably good measure of 

 its refractive and dispersive power ; so that there appears to 

 be no practical difficulty in moulding it into very minute 

 lenses. The realgar will retain its lustre, as we know by the 

 prism now mentioned, which we have kept for sixteen years. 



As the power of homogeneous illumination renders achro- 

 matic combinations of little use in microscopic observations, 

 the perfection of the single microscope must depend on the 

 degree to which we are able to remove the spherical aberra- 

 tion.' Hence the radii of the single lenses should be as 6 to 1, 

 and when they are of sufficient size, some of the contrivances 

 for a diaphragm within the lens should be adopted. 



Although we are not yet able to speak from our own ob- 

 servation of the excellence of Mr Pritchard's lenses, yet we 

 are in possession of the most satisfactory evidence of their im- 

 mense superiority to all single microscopes hitherto made, and of 

 their equality to the most expensiye Amician and achromatic 

 instruments. Mr Pond, our able astronomer-royal, having pro- 

 cured one of Dr Goring's improved Amician microscopes, with 

 metals of only six-tenths of an inch focu^, and three-tenths of 

 clear aperture, was desirous of comparing it with one of Mr 

 Pritchard's sapphire lenses. He accordingly selected a plane 

 convex sapphire lens of ^^jth of an inch in focal length, and he 

 found that, in every case, it exhibited all the most delicate test 

 objects that could be seen with the reflecting microscope, and 

 was otherwise equal to it in its performance. Since this com- 



