Dr. Goring on Achromatic Microscopes. 427 



IS accompanied by the proper calibre. I am not aware that 

 a magnifier or object-glass of ^V of an inch focus, and ^J-^ of 

 an inch of aperture, will perform any better than one of -^ 

 focus and -}-g of an inch of opening, which would possess the 

 same ratio or angle of aperture. As there are always 

 various degrees of vision in difficult objects of all sorts in 

 optical instruments, the said lines may be seen with more 

 or less facility with different object-glasses and instruments, 

 with more or less coaxing and management of the light ; 

 with more or less stability in the stand ; with more or less 

 accurate adjustment of the focus, &c. &c. 



There is, however, a way of determining which is the best 

 instrument, when two will show an object, but when there is 

 some doubt as to which shows it the best ; it is this : convert 

 the instrument into a magnifier for a solar microscope, and 

 see which will imprint the best image on a sheet of paper 

 (for such as the microscope is, such will be the image formed 

 by it). That which gives the strongest and most vivid pic- 

 ture will be sure to perform best when treated in this way ; 

 for it requires no little efficacy in a microscope to show a 

 difficult and delicate object fairly depicted on paper, all 

 faint and diluted rays being in this way utterly lost and ex- 

 tinguished. With the solar microscope I have alluded to 

 in the former paper, and the 0.2 object-glass attached to a 

 compound body giving it power equal to that of a single 

 lens of ^'5 of an inch, and the amplification of the image 

 in the camera being confined to 200, I have actually pro- 

 cured an image of the lines in the diamond beetle scales as 

 transparent objects^ and of course of every other variety not 

 more difficult than these; but with the 0.933 object-glass, 

 cceteris paribus^ I never could procure an image of any but 

 fig. 1, or of other lines of the same class and resolvable with 

 the same ease ; though the said 0.933 will show faintly 

 (when treated with an eye-glass only as a compound mi- 

 croscope) almost every object which can be seen by the 0.2. 



No difficult opaque object whatever can be exhibited by 

 the solar apparatus, such as any of the family of lines, as 

 opaque objects, the minutiae of a fly's foot, or of a mouse 

 hair, &c.^ though a most beautiful and pleasing picture of 



2F2 



