66 Description of' the Optiii^raph. 



of figures, wherein the elenicntarv comma (f-J-) is expressed 

 by 53950,3189 ; being an error but of one part lu sijcleen 

 thoiimrid eight himdred pcrts of a comma ! 



The practical appHcation of this will consist, in tuning 

 five successive fvwtlis up'.varus from the key or pitched 

 note, each quite perfect ; from the last note tuning down- 

 wards IwoJiJ'tks, and a major third in succession ; and con- 

 siderinu; the last note so tuned, as the equal temperament 

 ^fijth; from which 54ths are again to be tuned upwards 

 and 2V + in downward, for the second Vlh in this scale, 

 repeating the operation as above, and descending octaves 

 when necessary, until the twelfth Vth is compared with the 

 octave of the key-note, from which it will not differ in any 

 sensible degree, if the operation be rightly performed ; be- 

 cause 12 of mv fifths as above*, differ from 7 octaves but 

 -p^V o'^^'^ P'^'"'^ "^ '^ comma. 



f intend to publish the further details of this system when 

 leisure will permit; and am, sir. 



Your obedient servant, 



1 ^, Upper Crjnvn-strcet, Westminster, JoHN FaREY^ 



June 21, 1807. . 



IX. Description of the Qptigraph {invented hj the late 

 JSlr. Ramsden) as improved and made by ilir. Thomas 

 J>')NES, Mathematical, Optical, and Philosophical Instru- 

 ment JMaher, Ko. 124, Mount-street, Berkley-square. 



JL HE methods used to facilitate the practice of drawing in 

 perspective, as well for those versed in this polite art, as 

 for those who have made less proficiency, have been vari- 

 ous and numerous. Though some have supposed that the 

 warmth of imagination and luxuriance of fancy, which im- 

 pel the mind to the cultivation of the fine arts, are not to be 

 confined to mechanical modes, yet upon observation and 

 inquiry they will find that the most able and accomplished 

 arti&ts are often obliged to have recourse to some rules, and 

 to use some mechanical modes to guide and correct their 



* Which are eacli 2 t + 3 H, or -'^;~, and their /(aj. -8243988,5. 



pencil ; 



