102 UPON INSTRUMENTS CALLED PERISCOPIC 



nearly to the focus of all oblique refracted rays, exhibits uni- 

 versally the images with the greatest brilliancy and distinct- 

 ness. 



The exact curve of the surface of this board or table should 

 be that of a conic section, but the conca\e (spherical) one 

 answers sufficiently well. It is necessary for the reader un- 

 skilled in optics to know, that what opticians name the axis of 

 a lens, is that imaginary line that is supposed to pass through 

 its centre, and is not subject to any refraction, and all other 

 rays incident on the surface are refrangible, in proportion to the 

 angle they make with this axis, those rays impinging nearest the 

 centK* of the lens, and with the least obliquity of position, are, 

 refracted with the most perfect images, or with the least aberra- 

 tion, in double convex, plano-convex, and meniscus lenses. The 

 longitudinal aberration produces a focus short of the principal 

 one, and the lateral aberration a confused lateral extension of 

 the images, blended with prismatic colour. These aberrations 

 increase directly with the diameter and thickness of the lens, 

 and inversely with its focus. In lenses of large diameter, and 

 short foci, these aberrations will, by experiment, be rendered 

 very manifest, and which have been clearly demonstrated by 

 that learned optician, Mr. Benjamin Martin, in his Elements of 

 Optics, Dr. Smith, and others. 



Dr. Wallas- The subsequent paragraph, nacre 27, describes Dr. Wollas- 



ton's improve- , ,.,, • i • , 



ment, with the ton s proposed improvement : the substance, in his own words, 



meniscus de- is as follows. " The lens is a meniscus, with the curvatures 

 tation.* y tlU °" of its surfaces about in the proportion of two to one, so placed, 

 that its concavity is presented to the object, and its convexity 

 toward the plane on which the images are formed. The aper- 

 ture of. the lens is four inches, its focus about twenty-two. 

 There is also a circular opening, two inches in diameter, placed 

 at about one-eighth of the focal length of the lens from its con- 

 cave side, as the means of determining the quantity and direc- 

 tion of rays that are to be transmitted. The advantage of this 

 construction over the common camera obscura is such, that no 

 one who makes the comparison can doubt of its superiority ; 

 but the causes of this may require some explanation. It has 

 been aire dy observed, that by the common lens any oblique 

 pencil of rays is brought to a focus at a distance less than that of 

 the principal focus. But in the construction above described, 



the 



