252 SUPPLEMENT TO THE 



of the artificial illumination, by an incapacity of shewing 

 lined objects, except such as are of the lowest class, and 

 by giving very large spurious discs, with artificial stars ; 

 also by shewing easy test objects, with the lines faint, while 

 the spaces between them are darker and more opaque than 

 they ought to be. 



6. When the spherical and chromatic aberration is small 

 and faint, and the angle of aperture considerable, the lines 

 on proof objects become fine, sharp, and dark, and the 

 spaces between them clear and bright, (provided the 

 illumination is properly conducted :) they moreover become 

 visible in a very faint light ; if the instrument is perfectly 

 aplanatic, the outline and the lines are seen at once, and 

 the spurious discs of all brilliant points are very sharp and 

 small. 



7. An instrument, possessing an aplanatic pencil of 55 

 degrees, shews all the easy lined objects anyhow ; there is 

 no management whatever required to bring them out ; the 

 light may be thrown through them as directly as possible, 

 and still they will not disappear ; for in this case the pene- 

 tration of the optical part is so great as to get the better 

 of all imperfections in the illumination, and to exhibit the 

 object in a manner, in spite of the illumination, rather than 

 by its assistance.* 



8. Notwithstanding this fact, in order to obtain a maxi- 

 mum of distinctness and effect, it is in every case requisite 

 that the illumination should be of the most exact kind ; 

 and if the instrument has only an aperture just sufficient 

 to render a particular object visible, it is absolutely necessary 

 that it should be so, to render it visible at all. 



* Achromatic Illuminators, now so much admired by some observers, were 

 first proposed in the Micrographia, page 86. 



