692 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The method of illumination here referred to — which was originally 

 recommended by Mr. Stephenson for his binocular Microscope — has 

 in fact proved itself to be by far the best when it is a question of 

 using higher powers than about 300 times. It necessai-ily requires 

 very well corrected and properly adjusted objectives if the sharpness 

 of the image is not to suffer ; but if these conditions are satisfied, it 

 yields most striking stereoscopic effects even with objectives of 2 mm. 

 and less focal length, provided the preparation under observation pre- 

 sents within a small depth a sufficiently characteristic structure." 



We need hardly point out to microscopists the practical importance 

 of the suggestion for on the one hand retaining the advantage of wide 

 apertures to utilize the effects arising from parallax, while on the 

 other hand neutralizing the disadvantage of such apertures in the 

 loss oi focal depth. 



" Working Distance and its relations to Focal Leng-tli and 

 Aperture." * — Mr. E. Gundlach has an article on this subject, which 

 we regret not to be able altogether to follow, but we give his defini- 

 tions and conclusions : — 



" Working distance is the usual designation of the space between 

 the object and the objective on a Microscope when the former is 

 brought into proper focus ; or, in other words, when the objective is 

 brought to such a distance from the object that by means of the fonner 

 an air-image of the latter may be formed at a distance of 10 inches. 



" The working distance of an objective depends upon (1) the focal 

 distance, (2) the aperture, (3) the number of lenses of which the ob- 

 jective consists, (4) the proportionate curvatures of the lenses, (5) the 

 thickness of the lenses 



" The working distance of an objective may be expressed nume- 

 rically, from a comparison of the theoretically longest working dis- 

 tance as unity, and the result may be called the ' numerical working 

 distance.' 



" As a single lens without thickness cannot be produced, the actual 

 working distance of an objective will be much less than the unit of the 

 numerical working distance 



" If /is the focal length, a one-half of the angle of aperture, d the 

 actual working distance, n the numerical working distance of an 

 objective, then 



d 

 n = - -H cos a. 

 / 



* * * * 



"(1) If two objectives have equal focal length and equal 

 working distance but different angles of aperture, then the one with the 

 larger angle of aperture has the greatest numerical working distance. 



" (2) If two objectives have equal focal length and equal nume- 

 rical working distance but different angles of aperture, then the one of 

 the larger aperture has the shortest actual working distance 



"(4) The actual working distance of an objective is in direct 

 proportion to the numerical working distance." 



* Amer. Mou. Micr. Jouru., ii. (1S81) pp. 32-3 (2 figs.). 



