248 Critical Olservatiora ofi Dr. lVollaiion*$ 



point Out to your readers, what I consider to be the errw 

 of his reasoning, and the fallacy of his inference. 



In his description of the effect of the doul)le convex lens 

 in the common camera, page po, he states the known effect 

 of the images distant from the middle, or direct focus of 

 the lens, being somewhat indistinct, on account of the 

 plane of representation becoming, in distance, greater than 

 the principal focus of the lens, and the oblique pencils of 

 rays being refracted to a focus rather shorter than the prin- 

 cipal one. " On this account (he adds) it is in geneial best 

 to place the lens at a distance somewhat le'^s than that 

 ^vh^ch would give most distinctness to the central images, 

 because, in that case, a certain moderate extension is given 

 to the field of view, from Rn adjusinient better adapted to 

 lateral objects, without materiallv impairing tlie biightnes? 

 of those in the centre." The aberrations of the lens also 

 add to the indistinctness. 



The collateral indisliucfness in our portable chest ca- 

 meras is but trivial and uniu)portant ; and, in my opinion, 

 the remedy, as above proposed, will he found by the artist 

 to be worse than the defect, as the distinct and vivid centra! 

 images will be vitiated, and the extreme images but very 

 little improved. The most perfect remedy is that which 

 has been used by opticians in large cameras, for more than 

 fifty years, of placing a bottom board, or wintered tabic, 

 •with a concave surface, proportioned to the focal distance 

 of the lens; which, correspondini verv nearlv to the focus 

 of all oblique refracted rays, exhibits universally the images 

 with the greatest brilliancy and distinctness. The exact 

 curve of the surface of this board or table should be that of 

 a conic section ; but the concave one answers sufficiently 

 well. It is necessary for the reader unskil'ed in optics to 

 know, that what opticians name the axis of a lens, is that 

 imagmary line that is supposed to pass through its centre, 

 is not subject to any refraction, and all other rays incident 

 ■on its surface are refrangible, in proportion to the angle 

 they make with this axis; those rays in)piugiug nearest the 

 centre of the lens, and, with the least obliquity of position, 

 tire rei'racted with the most perfect images, or with the least 

 aberration, ip 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 co- 

 lour. These aberrations increase directly with the diameter 

 and thickness of the lens, and inversely with its focus. In 

 Icnsea of large diameter and short foci ihese aberrations 



will, 



