42^ Dl*. Goring 07i Achromatic Microscopes, 



lent proof objects. It is requisite for me to observe, that some 

 of them do not seem to be striated or furrowed with lines at all, 

 and there seems to be a good deal of difference in the difficulty 

 with which different specimens are resolved into lines. The 

 scales of many other brilliant foreign curculios also form ex- 

 cellent test objects, but not in general so difficult as these. 

 There seems to be nearly as much difference among them as 

 to the facility of revolution as among clusters of stars. I 

 have seen some so easily developed, and requiring so small an 

 angle of aperture^ as to be distinctly visible with the most 

 ordinary compound microscope. I shall take leave of this 

 class of objects by observing generally, that no accumula- 

 tion of condensed artificial light on them seems to have any 

 operation in supplying or compensating for want of aperture. 

 When we have an instrument possessing this one thing need- 

 ful in the requisite state of perfection for the purposes of 

 vision, it is surprising in what a faint light all the family 

 of lines may be seen, both as opaque and transparent bodies : 

 on the contrary, without this essential virtue in the instru- 

 ment used, we in vain look for lines even when we have ren- 

 dered our field of view in a complete blaze with condensed 

 rays. It is not mere light then that shows them, but the due 

 breadth of the visual pencil, with respect to the power em- 

 ployed. The Amician reflector (beyond comparison the 

 dullest and darkest under any given circumstances of the 

 whole family of microscopes) shows these said lines infinitely 

 better than any other instrument. The achromatics show 

 them very well also, especially the thin 0.3. Single magni- 

 fiers are very inert upon them as opaque bodies, because the 

 silver cups in which we are compelled to insert them to gain 

 the power of seeing opaque bodies do not easily admit of that 

 obliquity of illumination which seems one postulatum for 

 their exhibition. 



Equally unavailing to the demonstration of these objects is 

 mere magnifying power, however great even when obtained 

 from the deepest object-glasses, (though I have already had 

 occasion to remark that it is much more valuable, effective, 

 and penetrating when procured in this way than from the 

 operation of deep eye-glasses on shallow objectives,) unless it 



