ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



689 



Fig. 130. 



Fig. 131. 



and wide-angled objectives. In general appearance it is like an 

 ordinary objective. It bas an internal diapbragm, wbich is placed 

 directly under the posterior system of lenses, and is entirely con- 

 tained in tbe tube comprising tbe mounting, there- 

 fore avoiding any projection, and allowing tbe light 

 to enter only from below. By revolving the milled 

 ring of the mounting, the diapbragm is made to pass 

 laterally from the centre to the extreme edge of the 

 illuminator, thereby throwing rays of any desired 

 obliquity, between (central illumination) and tbe 

 extreme possible limit, 1 • 52 in crown glass. When 

 the diaphragm is at its extreme limit a second slit, 

 at rigbt angles to it, giving the same volume of 

 light, is opened by the further movement of the 

 milled ring, thus utilizing two pencils at right angles. 

 The illumination is said to be amply sufficient with 

 the highest powers, and the fact that it is used with 

 only central illumination of the mirror, will, it is con- 

 sidered, " prove especially valuable to those who do 

 not possess instruments with the modern swinging 

 substage and mirror-bar." 



The illuminator is also said to give excellent 

 results when used as non-immersion. A cap with 

 minute aperture (Fig. 131) to facilitate centering, and an adapter 

 (to receive the optical part without the diaphragms and so to give full 

 aperture) accompany it. 



Gundlach's Immersion Condenser.* — E. Gundlach discusses this 

 subject, and expresses the opinion that of all the apparatus for pro- 

 viding oblique illumination for large apertures, the Abbe condenser 

 bas apparently been the most efficient, and has been generally adopted 

 as tbe most suitable illuminator for tbe widest angled objectives, 

 hence it is advisable to inquire whether this form of condenser is 

 capable of doing all that is demanded of it now, or that will be 

 demanded in the near future ; and to this inquiry he has given much 

 special study. As the full advantage of a very wide-angled objective 

 cannot be had unless light can be made to pass through any part of 

 its aperture at will, the Abbe condenser would be tbe best, if it were 

 possible, practically, to increase its angle to correspond with that of 

 the objective ; but it can be shown, Mr. Gundlach considers, that it 

 cannot be so increased, and that it cannot approach within 20° or 

 more of 1 ■ 52 N. A., as is now, or soon will be, desirable. 



" If the point where the optical axis of the objective cuts the plane 

 of the object be considered the vertex of an angle which has the ex- 

 tended optical axis of the objective for one side, then the other side of 

 the angle extended downward will cut the under side of the slide on 

 which the object is mounted, at a certain distance from the axis, and 

 this distance is proportional to the thickness of the slide. Besides, if 

 tbe said angle is equal to half the angle of aperture of tbe objective, 



* Amer. Mon. Micr. Journ., iii. (1882) pp. 85-7 (1 fig.). 



