260 Dr Goring on the Construction of the 



adopting erecting eye-pieces of my construction, will very soon 

 be recognised even by an English optician. As to increasing 

 the angular aperture of the primary or telescopic object-glass, I 

 only say, that if it is well executed, its defining power will be 

 increased, as will be shewn by its giving a much smaller spuri- 

 ous disc to a star than one of a longer focus and smaller angle 

 of aperture will ; and if it is composed of two double object- 

 glasses with their inner curves cemented on the French plan, 

 it will be more luminous than an ordinary triple one. 



I here give the construction of a secondary object-glass, or that 

 belonging to the erecting eye-piece as made on my plan, and at 

 my instigation, by M. Charles Chevalier, 163 Palais Royal, Pa- 

 ris ; and if it is thought that it consists of too many glasses, 

 and that the quantity of light lost will be very great, I can safe- 

 ly affirm that whoever tries it, provided the convex lenses are 

 made of fine French plate-glass, will be agreeably surprised at 

 its luminousness. With an object-glass of two French inches 

 of aperture we may have a power of 40, with sufficient light, 

 Avhich I should say is as much as can be used advantageously 

 upon any distant objects, even in a much clearer atmosphere 

 than that of the British Isles. I may observe that the said se- 

 condary object-glass may be made of two double achromatics 

 instead of three, but then the edges of the field of view will not 

 be quite so good, if it is a large one at least. 



My readers have only to unscrew that part of an erecting eye- 

 piece which forms the second image, consisting of two glasses, usu- 

 ally called the third and fourth, with a stop having a very small 

 perforation in it, which is placed between them, and to suppose 

 two double cemented achromatics substituted for the fourth, and 

 another for the third having the same foci and apertures with 

 the common glasses, the stop being removed, and they will 

 arrive at a perfect idea of my construction. That M. Che- 

 valier has executed for me, has the same dimensions, foci, &c. 

 as the secondary object-glass of a 20-inch day and night glass ; 

 and the improvement I wish to suggest upon it consists merely 

 in placing the third glass in a sliding tube, so that it may either 

 act positively or negatively, either in contact with, or at a dis- 

 tance from the fourth, for in this w^ay the power of the telescope 

 may be augmented or decreased one-half, and changed, say 

 from 40 to 20 or from 20 to 10, or the reverse, and every in- 



