74 Dr Goring on Monochromatic Light, fyc. 



a bad field of view, that is to say, the vision is only perfect in 

 the centre. In refracting instruments, a good deal may be 

 done by combining several object-glasses together, more espe- 

 cially if they have intervals between them, (which, however, 

 produce great inconvenience, by shortening the anterior con- 

 jugate focus, and thus precluding the use of high powers.) 

 As the action of metals is very simple in comparison to that 

 of lenses, I shall give an illustration of the mischief occasion- 

 ed by what is called improving the oblique pencil in the Ami- 

 cian reflecting microscope. It must be evident, that, as the an- 

 gular aperture is increased, the error of the oblique pencil 

 must also be augmented ; for it will come more and more ob- 

 lique. Now it has actually been recommended to opticians to 

 render the metals of the aforesaid instrument spherical, be- 

 cause in this case every pencil becomes equalized, and may 

 be said to be equally bad or good, distinct or indistinct, 

 throughout the whole field of view, though, so far as the ob- 

 jective part of the instrument is concerned, the whole instru- 

 ment becomes utterly worthless if this figure is adopted. 

 Those who work the metals of this instrument well know what 

 infinite pains they have to get rid of this spherical figure and 

 to attain the true one, * and that when they have done so, the 

 point is effected at the expence of a certain want of perspicuity 

 about the edges of the field of view, which is altogether irre- 

 mediable with low powers, which take in a large portion of the 

 marginal part of the image. If Gregorian and other telescopes, 

 having a large angular aperture, are charged with a low power, 

 the same defect is also very perceptible in them. 



Of a piece with the performance of a spherical metal is that 

 of a globular or bird's-eye object-glass, which is considered to 

 produce an excellent correction of the oblique pencils, be- 

 cause they come exactly like the centrical ones, which are ut- 

 terly uncorrected. 



When the omnipotence of analysis shall point out some 



* I have known Mr Cuthbert to have been employed a whole week in 

 getting rid of nothing but the spherical error of one of his metals of ^ths 

 of an inch focus and the same aperture, and not able to succeed at all in 

 certain states of the weather, supposing always that a perfect figure was 

 to be combined with a perfect polish. 



