of Opaque Objects^ and on a Focimeter. 431 



nerally underrated by opticians. To prove mine, I made what 

 may be called a Focimeter. It consists of a flat piece of brass, 

 which fixes on the arm of the microscope (fig. 5.) ; at the end 

 is a female screw that has exactly fifty threads to the inch, 

 to which is fitted a male screw about half an inch long, with a 

 hole drilled quite through it, and a large ivory head (fig. 6.), 

 with twenty divisions fixed on its lower end ; consequently one 

 division on the head is equal to the l-1000dth of an inch. After 

 the end of the screw and the brass plate are ground together on 

 a flat hone, there must be glued over the hole in the male 

 screw, a small piece of the outer membrane of the eye of the 

 Libellula grandis, and a piece of very thin foil with a small 

 hole in its centre laid over the female screw. When used, fix 

 it to the arm of the microscope, and throw the light up the 

 hole by the mirror ; lay the highest power on the foil, turn the 

 screw up till it just touches the foil, and set down the number 

 of turns and divisions that it takes to bring the object down 

 to the focus of the lens. 



Proceed in the same manner with the rest of the powers ; 

 they may then be easily reduced to the nearest fraction having 

 one for a numerator : by this means I found that my highest 

 power, which I considered l-60th of an inch focus, was only 

 1-4 6th inch focus, and the rest in proportion. I have made a 

 double convex lens, which is the smallest I have ever seen, and 

 is only l-100dth inch focus, as it took but half a turn of the 

 screw to bring the object down to the focus. The thickness 

 of the foil may be neglected even in the highest powers ; as it 

 does not amount to the l-1000dth of an inch. 



Dr. Goring uses the scales of the Lepidoptera as test-ob- 

 jects: but the most beautiful and delicate test-objects that I have 

 ever seen are the scales of the Lepisma sacc/iarina ; they are 

 so very thin, and the lines upon them are so very fine,that they 

 will bear almost any power. The beautiful green convex scales 

 of the small English diamond- beetle, which is very common in 

 the summer months, is a very good opaque test-object. 



The focal distance of large lenses for telescopes, &c. is reck- 

 oned from the centre of the body of the lens (under half an 

 inch focus) ; but in my opinion the focus of microscopic lenses 

 ought to be computed from the surface next the radiant ob- 

 ject, otherwise another operation is necessary; viz. taking the 

 thickness of the lens and adding half of it to the focal distance : 

 but as small lenses are so very apt to be lost or cracked, it is 

 much safer and better to reckon from the surface. 



I beg to remain yours, &c. 



Ninesham, Oct. 23, 1828. G. DaKIN. 



LXXI. On 



