ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 487 



L., A. P.— Inquiry as to the best proportion of Eye-piece to Objective. 



Engl. Mech., XLVII. (1S88) pp. 169-70. 

 Objectives, English and Continental. 



[Inquiry "how to compare the Euglish and Continental nomenclature of 

 objectives." 



Keply by " T. F. S." that such a list as desired " would be impossible, from 

 the simple fact that the magnifying powers of the objectives as supplied by 

 the makers do not agree with their own catalogues." He then proceeds as 

 follows : — 



" Within certain limits, however, the focal distance of the objective is not of 

 the slightest importance, the numerical aperture being the only measure of 

 its capacity to show fine detail. Thus if a 1/4 in. and a 1/8 in. objective 

 have the same N.A., and an object is shown under the first with an eye-piece 

 maguifying eight, and under the latter with an eye-piece magnifying four 

 times, there will be no difference between the images whatever, neither in 

 brightness nor in the amount of detail shown, provided both glasses are 

 equally corrected. The measuring the capacity of an objective by its focus 

 is an old superstition handed down from the time when the angle had to be 

 limited from a want of skill in making the necessary corrections for chromatic 

 and spherical aberration, and like most superstitions, has lingered for a long 

 time after the cause, which made it a real faith, has disappeared. I can only, 

 then, counsel 'A Constant Reader' to throw aside all notions of measuring 

 the capacity of a glass by its focal length, and in its place to study the 

 'Aperture Table' given in the 'Journal' of the Royal Microscopical 

 Society, where he will find the resolving power for any given aperture, and 

 can then compare catalogues for himself." 



It cannot, however, be quite so broadly laid down that the " focal distance of 

 an objective is not of the slightest importance." Even when resolution is 

 alone considered there is a proper relation between aperture and power 

 which renders a knowledge of the latter important.] 



Engl. Mech., XLVII. (18S8) pp. 125 and 146. 



(3) Illuminating and other Apparatus. 



Dumaige's Camera Lticida. — The peculiarity of M. Dumaige's camera 

 lucida is that the prism and reflecting mirror are in a box, which can be 

 closed when the camera is not in use. When in use the cover of the box 

 hangs down at the side of the eye-piece, as shown in fig. 76. The optical 

 arrangement consists of a small prism over the eye-piece, covering half 



Fm. 76. 



the eye-lens, with a mirror about 1 in. square which receives the imago, 

 of the paper and pencil and reflects them to the prism, whence they 

 are reflected to the observer's eye, which views simultaneously the image 

 from the object through the uncovered half of tho eye-lens. The 



