16 Ohjed-glasses and their Definition. 



IV. — Ohjed-glasses and their Definition. 



By F. H. Wenham, Vice-President E.M.S. 



OuE thanks are due to Dr. Pigott for his laudable attempts to 

 advance the microscope object-glass, but as he appears to stand 

 forward as the pioneer of a new era in their construction, ignoring 

 all glasses made before the publication of his first essay as " old- 

 fashioned," such dictatorship naturally challenges inquiry as to the 

 merits of his investigations, and whether they have in any way con- 

 tributed to the end in \iew. 



His papers published in the July and September Nos. of the 

 'Monthly Microscopical Journal' for 1870, only tend to mystify 

 the simple action of the immersion lens, for he therein endeavours 

 to show that there are some new and hitherto unnoticed optical 

 conditions peculiar to the immersion front, and brings forward the 

 remarkable error that, if various fluid media, such as water, tur- 

 pentine, &c., are introduced between the cover and front lens, a 

 larger aperture or greater angular pencil of rays is transmitted from 

 a radiant jjoint or object, ichen mounted in balsam, but in fact the 

 angle must necessarily be limited on such objects. 



In the year 1854 Professor Piobinson and myself* demonstrated 

 simultaneously that the angle of illumination, and also the aperture 

 of the object-glass, were reduced on objects mounted in Canada 

 balsam : Professor Piobinson assigned the limit as 81'^, I stated it at 

 82^. This slight difference refers to the refractive index of the 

 glass cover, which was not taken as the same in each case. The 

 pencil must of necessity be confined within the angle of total re- 

 flexion; this may vary from 40^^ to 41"^ 30", according to the 

 density of the crown-glass cover. This so far is an established 

 optical fact. Dr. Pigott's inference is, that by the introduction of 

 fluid films of various refraction, between the front lens and cover, 

 more or less of the entire aperture is again recovered and utilized, 

 as the total reflexion of the upper surface of the cover is then pre- 

 vented, or is extended definitely according to the refi-active index of 

 the interposed medium employed, and that a greater angle is trans- 

 nutted in consequence. This assumption is taken quite regardless 

 of the dimensions and construction of the component lenses of an 

 objective, and their capabihty of admitting rays in excess of a cer- 

 tain angle. The paper is prefaced by the followuig remark, — " One 

 might almost venture to declare that perhaps too much exclusive 

 attention has been paid to the objectives, to the neglect of the pen- 

 cils radiating from the object . . . the primary behaviour of the 

 tiny sjiray of rays is of the last importance to the final definition." 



* ' Quarterly Journal of Microscopical Science,' July, 1854, p. 212 ; ibid., Jan., 

 1855, p. 165. 



