3o8 On the ihubU Refra6iion nf Rock Ciyjlal. 



method I piirfucd to mcafiux- the deepcft wells and the longcft 

 galleries. 



I Hiall eonclade this note with a fiiccinfl; account of fpme 

 experiments which I ninde with the ifofceles prifm of rock 

 cryflal already mentioned. By applying it to a finiple ca- 

 mera ohfcura, the objerts painted in an inverted pofition, when 

 this prifm is not ui'ed, are made to appear in their proper' 

 fituation. By adaplino- it to alironomieal telefcopes, the fame 

 eff'ecl will be produced in regard to ohjecls, which, when feen 

 through the two convex giairesthatcouipoie it,api)ear inverted. 



This prifm fupplies the means of (horlening telelcopes de- 

 ftined to view terrellrial objufts, becaufe, by employing it 

 with a convex cve-glafs only, and an ohjecl-glafs fimple or 

 achromatic, inftead of three, ionr, or five eve-glalles, objefts 

 whicli without itsinterpolilion would have appeared inverted, 

 will be feen in their proper pofition. By thefe means, one, 

 two, three, or even four eye-glafles would be faved, and the 

 teleffope might be iliortened by nearly the whole length oc- 

 cupied by thefe eye-glalfes. Light alfo will be gained; for 

 this prifm occalions no lofs in this refpeft, the matter of 

 .which it is formed being excecdinii'ly tranfparent; nor will 

 any thing be lo(t in regard to diilincfnefs ; for, as the prifm 

 . is very near the eye, the faults which might arilc from any 

 inexaofnefs in the polifliing of its two furfaces will not be 

 ienlible. 



In employing this tclcfcope it miift be remembered, that 

 at the fame time that it inverts objects by turning them up- 

 lide down, it turns them from right to left, and what appears 

 . on the right in the licld of the telefcope is really on the left : 

 for example, if a man going from right to left be viewed with 

 it, he will appear in the telefcope as if going from left to 

 right, but in his natural filualion : whereas, if viewed with 

 the lame telefcope, taking from it the prifm of rock cryltal 

 and leaving only the cye-glafs which inverts the objecls, the 

 man would appear not onlv to he proceeding in a direftion 

 contrary to thai in which he reallv is advancing, but he would 

 appear alio inverted — the common eft'eft of telelcopes whicU 

 have onlv a eoitvex cyeirhifs. 



Expccicnce alone can iliow whether this method of fliort- 

 eniiig land telefcope> can be as ui'eful as it ap|)ears curious, 

 and whether it will be poflible to conftruft one at fuch a 

 moderate price as to maintain a competition with connnon 

 telelcopes, without which this inlirument would remain 

 aiyong the number of thofe difcovcries which are rather 

 curious than ufeful. 



XLIX. Prc^ 



