On the Strengths and Values of Spirituoiis Liquors. 329 



more completely will the indistinctness of lateral objects 

 be avoided *. 



According to this principle all spectacle glasse^ should be 

 convex on their exterior surface, and concave within. The 

 section of those for long-sighted persons will assume the 

 form of a meniscus or crescent. Fig. 2. and those adapted 

 for short sight \\ ill have their principal curvature on the 

 concave side. Fig. 3. 



It is only necessary to add, that the advantage of this 

 improvement in the form of spectacle glasses has been 

 confirmed by a sufficient number of experiments on different 

 persons, and that those in particular, who are very long, or 

 very short sighted, are much benefited by them. 



The most advantageous proportions of curvature for ob- 

 taining the different focal lengths, now generally distin- 

 guished by certain numbers, have also been duly considered, 

 and the manufacture of spectacles on this construction has 

 been undertaken by Messrs. P. and J. Dollond, to whom 

 the exclusive sale of them is secured by patent, and whose 

 well-known skill in the construction of optical instru- 

 ments ensures to this improvement every advantage of 

 correct execution. 



The opportunity afforded by these glasses of looking rovnd 

 at various objects, it is thought may not improperly be ex- 

 pressed by the name of Periscopic Spectacles. 



LXII. Of the general Relation letween the Specific Gravi- 

 ties Q7id the Strengths and Values of Spirituous Liquors, 

 and the Circumstances by which theforvier are influenced. 



[Concluded from p. aio.] 



Problems and Rules for the Adaptation of Mr. Gilpin's 

 Tulles to the present Standard. 



§ 31. Problem \.—The specific gravity of a liquor at 

 any given tempei-ature being given, to find that which it 

 possesses at any other temperature. 



* To mathematicians it will be evident that any ray which does jict 

 pass through the centre of a lens cannot be at right angles to both sur- 

 faces ; but they will also perceive, that when any jw«// oblique pencil 

 makes equal angles with the two surfaces of a thin lens, the inclinatioa 

 a u to each i. so small, that its focal length B D, Fig +• will not sen- 

 sibly differ from A C, that of a central pencil. 



Practical 



