308 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Aperture therefore, as meaning distinctly " opening," may be pro- 

 perly defined by reference to the diameter of the pencil (at its emer- 

 gence from the back lens) which the objective has taken up from any 

 given point of the object and collected to a focus at the conjugate 

 point of the image. Not, of course, the absolute measure of this 

 diameter or " opening," for that would class a 1-inch objective as of 

 larger " aperture " than a ^-inch, but the relative opening — that is, 

 the opening in relation to j)Ower or " focal length." 



Thus, if two objectives are of the same power, the one that has the 

 larger opening — that is, the one which transmits from the object to 

 the image the wider pencil — has the larger ajierture. If, however, 

 the two objectives are of different powers, then the one which has the 

 wider pencil relatively to its focal length has the larger aperture. 



If Fig. 51 represents diagrammatically an objective of given 

 power (or focal length*), its aperture is obviously reduced if a stop 

 is inserted at the back of either of the lenses. The power remaining 

 the same, the aperture varies with the emergent pencil. 



Fig. 52. 



Fig. 51. 



Fuj. 53. 



The case of different powers and the same or different emergent 

 pencils is shown in Figs. 52 and 53. 



If an objective of lower power (see Fig. 52) is compared with the 

 previous one (indicated by dotted lines), the emergent pencil may 

 remain the same, but the aperture is obviously smaller in the case of 

 the lower power. 



If an objective of twice the power of the first is taken (see Fig. 53), 

 the emergent pencil may be only half the diameter, but the power 

 being doubled, the aperture remains the same. 



(4) Increase of Aperture with the increase in the density of 

 the Medium.— Apertures exceeding 180° angular in air. — It is, of 

 course, common ground with both theories of aperture that when 

 the medium remains the same, as in the case of dry objectives, the 

 larger the angle of the admitted pencil the larger the aperture, a 



* In these diagrams there is, of course, no line which represents visibly the 

 " focal length " of the objective, as in the case of a single lens. In a compound 

 objective the " focal length " is arrived at by comparing the objective with a single 

 lens of identical power, and the expression of the focal length of the single lens is 

 taken as that of the objective. 



