ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. Ill 



"Swinging Substage," or " Swinging Tail-piece." — At the time 

 this contrivance was first introduced it was known as a " Swinging 

 Tail-piece," but since that time the term " substage " has been 

 almost universally substituted. The earlier name is obviously, how- 

 ever, the more appropriate, as it is not simply the substage which 

 swings, but the mirror also, and we intend to adopt in future the 

 expression " swinging tail-piece." 



Value of Swinging Tail-pieces. — In addition to the opinions 

 cited at p. 666 of Vol. I. (1881), the following has been published 

 during the past year : — 



: Mr. J. D. Cox, in the paper above referred to (see p. 102), 

 considers that the swinging of the mirror-bar on the optical centre of 

 the instrument is a positive improvement, but that the swinging of 

 the substage is of very doubtful value. " In the former case several 

 real advantages are gained. First, the mirror is kept at its proper 

 focal distance from the object. Second, it may be swung above the 

 stage for illumination of opaque objects. Third, it allows the instru- 

 ment to be used for measuring aperture of object-glasses, by converting 

 it into Smith's ' Universal Apertometer.' * But when we ask for 

 the advantages of swinging the substage with illuminating apparatus, 

 it is difficult to find them. It is plain that we don't want to swing 

 the polariscope, the parabola, the dark wells, the Webster condenser, 

 the wide-angled achromatic condenser, or the immersion illuminators, 

 and could not if we would, for the form and mounting of these acces- 

 sories is inconsistent with doing so. The question must practically 

 be narrowed to the desirability of swinging the diaphragm and the 

 low-angled achromatic condenser. Of course none of the flat dia- 

 phragms can be swung in this manner, and no advantage seems to be 

 found in the use of the sharp-nosed diaphragms with oblique light. 

 The fact is that there are advantages in taking oblique light directly 

 from the mirror ; for the chromatic fringes at the margin of the illu- 

 mination often enable the microscopist to modify the light in a way 

 to get increased resolution by turning the mirror so as to take the 

 most lateral rays and those nearest the blue end of the spectrum. 

 More range in quality of illumination can be got by the practised 

 hand in this way than by the oblique use of the diaphragm. 



" In the use of an achromatic condenser, it must be a very low 

 angle indeed which will work far enough from the bottom of the 

 stage to allow much swinging to right or left, especially when we 

 take into account the fact that the centering of the substage 

 becomes more important when it is swung away from the axis of the 

 instrument. 



" The centering arrangement of the substage will occupy so 

 much lateral room that it can be swung but a little way before 

 striking the stage. Again, any achromatic condenser of even 

 moderate angle can be sw^ung very little to right or left before its 

 marginal rays will become parallel to the bottom of the slide con- 

 taining the object under examination, and they then, of course, cease 



* See tills Journal, ii. (1879) p. 775. 



