260 Dr. Goring' s Commentary on 



by every other person here who inspected it. I may observe, 

 that Professor Schumacher is not more difficult to please in 

 these matters than other men, and has, now several sapphire 

 lenses of Mr. Pritchard's own recommendation, which he con- 

 siders very fine. I shall only say, that Dr. Wollaston acted 

 with admirable consistency in spurning a good diamond lens, 

 and selecting a bad sapphire par excellence. It is no discredit 

 to Mr. Pritchard, or to any man, to make bad lenses of the 

 precious stones, for they are subject to the defect of flaws, im- 

 perfect consolidation, and crystallized texture, which unfits 

 them for optical purposes (like glass, when caused to cool 

 very slowly, and thus to assume a crystallized form). If three 

 parallel plates of sapphire are cut truly in the axis of the stone, 

 the middle one, perhaps, will be good for nothing, and the 

 other two will make excellent lenses : it is impossible to know, 

 a priori, what part of the stone is good or bad, except by 

 polishing and trying the laminae*, or making them into lenses 

 outright, which latter method is generally preferred by Mr. 

 Pritchard. The hardest stones make the best lenses ; for these 

 reasons, I am afraid it will be for ever impossible to make 

 lenses of any considerable size of any of the precious stones. 

 However, a lens of sapphire, about an eighth of an inch dia- 

 meter, can be procured free from all defects. In order that 

 jewelled microscopes may not lose their reputation, I would 

 counsel all those who choose lenses of any of the precious 

 stones, to try them as object-glasses, as it will be utterly impos- 

 sible that any defect, either with regard to their giving double 

 images, or that muddiness or indistinctness arising from a 

 crystallized texture, can fail to be visible in this way. It has 

 been supposed by some, that though a lens has, properly 

 speaking, only one axis, it may be considered to have a great 

 number of eccentrical or oblique pencils, which must necessa- 

 rily be inclined to the axis of single refraction^ and thus inevi- 

 tably cause double images, however truly the axis of the lens 

 may coincide with the axis of single refraction. I have nothing 

 to object to this in point of theory, but in practice it certainly 

 does not occur : for instance, if we cause a sapphire . leps, U> 



* The crystallization may generally be seen with a good engyscope of consi- 

 derable power, by using oblique candle-light to illuminate the stone. 



