INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 887 



not follow that, admitting a certain amount of penetration to bo useful 

 in the eye, the same is true with the objective. 



As to the view that objectives with penetrating power enable us to 

 see the parts of objects in their true and natural relations, and that 

 the greatest part of histological work is being done with them to-day, 

 he replies, " the more is the pity, because most of it will have to be 

 done over again with better lenses." The fallacy that such objectives 

 enable us to see different planes of objects in their true relations 

 arises from confounding depth of focus with stereoscopic effect, the 

 latter not being dependent upon the former. Diagrams are given of 

 two pieces of wire netting, in squares of different patterns, which are 

 supposed to be laid over one another. " With a corrected objective 

 you see the upper one first, and following nature's plan with the eye, 

 you focus down through it and see the other. If both are seen at once, 

 as a penetrating objective would do, we get a compound figure totally 

 unlike either — an illusion of sight." 



ToUes's Improved Traverse-lens, Illuminating and Aperture - 

 measuring Apparatus. — Mr. Tolles has improved upon the traverse- 

 lens which he described in 1879,* and his new apparatus is shown in 

 Fig. 103. The following description is supplied by him : — 



The apparatus being intended for measurement and use of the 

 largest apertures, a nearly semicircular sector-plate became a neces- 

 sity. 



For more convenient use, a small stage c is supplied, but this stage 

 and its accompanying traverse-lens h are readily removable. The 

 traverse-lens h is less than a hemisphere by the thickness of an object- 

 slide — assumed 'OS inch. 



For convenience of mounting in its cell, the top surface of h 

 has a curvature moderately convex, but with a medium of the index of 

 glass connecting the lens and slide, the cuxwature is neutralized. 



When the object to be viewed is in position, it is of course in the 

 centre of motion of the illumination apparatus, as guided by tlie groove 

 in the plate A. The semicircular rim of the plate is graduated to 

 degrees, and numbered each way from the zero point (midway from 

 the ends) to nearly 90°. 



When the truncated cone of glass Tc, having immersion contact 

 witli the traverse-lens h, is moved from zero to the degree of obliquity 

 where the light fails to give view of the object thrtjugh the objective, 

 at tlie eye-piece of the Microscope, then the half angle of interior 

 aperture can be noted by means of an index at the edge of the plate h. 

 If desirable, the half angle on the other side uf tlie axis can be ascer- 

 tained in the same manner, the cone h being first transferred to the 

 fitting on which the prism n is shown in the figure. 



The outer end of the cone h can be cut ott" by means of a cup z, 

 with a semicircular oi)ening so as to limit the light to an axial direc- 

 tion in the cone. For greater accuracy the apparatus includes an extra 

 arm j> and p', for carrying a small candle in tlie tube / as a radiant, 

 and which attaches to the plate h in place of the prism m. This arm 



* Sec 111 is Journal, ii. (1879) p. 388. 



