Royal Microscopical Society. 51 



II. — On an Aerial Stage Micrometer: an improved form of 

 engraved "Lens-Micrometer" for Huyghenian Eye-pieces, 

 and on finding Micrometrically the Focal Length of Eye-pieces 

 and Objectives. By Gr. W. Koyston-Pigott, M.A., M.D., &c. 



(Bead be/ore the Eoyal Microscopical Society, Bee. 4, 1872.) 

 (Continued from p. 5.) 



In the present article it is intended to give an easy method of de- 

 termining the focal lengths of either eye-pieces or objectives ; and 

 also their magnifying power micrometrically. 



It is indeed generally known that the focal length of an eye- 

 piece may be found by multiplying the focal lengths of each com- 

 ponent lens, and dividing the product by their sum when diminished 

 by the interval separating the lenses, 



or f=/x/'-K/ + /'-<0. 

 But then the observer is landed in the double difficulty of finding 

 these focal lengths and the interval. The method I now propose 

 obviates these measurements altogether, and it entirely resolves itself 

 into micrometric measurements. 



In the paper given at page 266 (No. XL VIII.) the writer gave 

 methods for establishing the focal lengths of a single lens by means 

 of ascertaining the magnifying power when a certain distance inter- 

 venes between the object and image. 



For deeper glasses, as ^ inch, J, &c, it will be found sufficient to 

 divide the distance by the magnifying power when increased by the 

 number two. 



It has been for a very long time a custom with opticians to place 

 a rule, divided into inches and tenths, on the stage besides a glass 

 micrometer divided into hundredths ; when, by observing with the 

 left eye how many inches and parts are subdivided by a magnified 

 hundredth in the field of view observed with the right, a tolerable 

 estimate can be roughly formed. Besides, however, the fact that 

 both eyes frequently have different distances of distinct vision, and 

 that several experiments give different results, it can lay no claim to 

 the accuracy of a scientific determination. 



The new method may thus be described : — To find focus of an 

 eye-piece, measure by a stage micrometer, divided into lOOths and 

 lOOOths, the image of a ten-inch circular white disk placed on a 

 black board 100 inches from the stage of the microscope, as minia- 

 tured by the given eye-piece used as a condenser* 



Let N be the size of the image in number of thousandths — so 

 that m is the proportionate size of object to image, or 10 -f- N — 



Then there is a beautiful relation between the focal length and the 



* The eye-piece is arranged with its field-glass towards the disk. 



