Aplanatic Object-glasses. 249 



I here, then, enter upon the discussion of the merits and 

 demerits of the objectives of the said instrument*, these 

 being much more perfected than those of another, of previous 

 make, which I saw in the possession of Mr. Howship, of Great 

 George-street, Hanover-square, to whom I received a letter of 

 recommendation from Mr. Spilsbury, of Ball-Haye. To the 

 signal politeness of these gentlemen, in furthering my views, I 

 Jim greatly indebted. 



Four object-glasses accompany Messrs. Chevaliers' instru* 

 ments (at least those marked perfectionnes,) usually rated at 

 the following foci : 14 French lines, 10 ditto, 4 and 4 : the twO 

 Jatter combine together at will, and give a focus of two lines. 



14.) Focus about 1.42 of an English inch, clear aperture 

 0.31, original aperture as reduced by a stop, 0.10. ' vii^t 



It is perfectly achromatic with its clear aperture, and may 

 be used without a stop on most transparent objects ; requires 

 to be cut off to 0.23, to give the necessary distinctness for 

 opaque ones. — (AVhen I speak of the apertures which C.'s lenses 

 will bear, I must be understood, here and elsewhere, only with 

 regard to the middle of the field of view y or rather that part of 

 it where the distinctness is greatest f, for double object-glasses 

 give the central rays only correct, and confuse the oblique one^ 

 very much„ for which reason, conjoined with the small aper- 

 tures they admit of, they were abandoned by Mr. Tulley, 

 for the triple construction, the true and regular form for the 

 microscope.) — There is an excess of spherical aberration in 

 convex lenses ; neither are the glasses well ground, or centered, 

 or duly adjusted. The concave of this object-glass is tar- 

 nished, and there are traces of seediness in the cement, which 

 is, indeed, to be seen more or less in the whole of them. 



10.) Focus about 0.91, clear aperture 0.23, original stop 

 0.09. 



» The objects employed by me in looking into the defects and excel- 

 lencies of these glasses, were an artificial star, and a piece of enamelled 

 dial-plate, the phenomena presented hy which, when put out of focus ^ 

 incontestably warrant the judgment I have pronounced upon them, as 

 any true optician will admit. 



t When an object-glass is out of adjustment, its maximum of distinct- 

 ness is not in the centre of the field of view, but somewhere else, accord- 

 ing to circumstances. 



