240 Dr Goring 1 s Remarks on M. Chevalier '■? Paper, 



gular aperture of fifty degrees or thereabouts. Each of its com- 

 ponent lenses has some defect or another, which renders it unfit 

 to act alone, or, in short, in anv other manner than combined 

 with the other two in the required order. The composition of 

 the three is, however, without fault or blemish of any land, it 

 cannot be said that there is any defect of centering or adjust- 

 ment, or any spherical or chromatic aberration, which the eye 

 at least can detect ; for that tendency to over-correction, both 

 for sphericity and dispersion, which seems irremediable in ob- 

 ject-glasses, embracing a very large pencil, is on this very mi- 

 nute scale wholly insensible, and so of course is the secondary 

 spectrum, so that M. Chevalier has here contradicted himself 

 by his own works, and produced that ultimatum which he insists 

 upon being unattainable. Moreover the oblique pencil is well 

 corrected. 



It will be recollected that the aberrations of an ordinary un- 

 corrected object-glass of the same focus and aperture are faint 

 in comparison to those of longer foci, taking in a pencil of the 

 same size, so that a common lens of i\jth of an inch focus, and 

 an aperture of thirty-six degrees, has the power (though with 

 much confusion,) of showing all those lined objects which an 

 achromatic of one inch focus and an equal aperture can show 

 when raised to the same power, (let us suppose = <,\.\\ of an 

 inch). Therefore, if we wish to make a fair comparison between 

 the performance of an achromatic object-glass and a common 

 one, both should be of the same focus and angular aperture. 



Though we may in some measure make a common compound 

 microscope compete with an achromatic one, having an object- 

 glass of long focus, by making that of the former very deep ; 

 yet when we get a perfect achromatic system of ^th of an inch 

 focus, its superiority over a common ^th of an inch becomes 

 nearly as manifest as that of an achromatic of one inch focus 

 over a common one inch one. 



M. Chevalier has now, I think, arrived at the shortest focus 

 which it will be possible to use upon opaque objects. With re- 

 gard to the performance of this system, I think it enough to 

 say, that I have seen with it the two sets of diagonal lines on 

 the true difficult Picris biassicoc, which I never saw before, 

 with any sort of certainty, with any achromatic object-glass ; 



