ZOOLOGY AND BOTANY, MICROSCOPY, ETO. 647 



The tube-slide is screwed to the bottom of the body-tubo. The plane 

 of the sliding motion is purposely made, not at right angles to the axis of 

 the instrument, but inclined at an angle to it, so that the objective falls and 

 rises as it is inserted or withdrawn. In this way any danger of contact 

 with the object is avoided. The objective is screwed to the objective-slide, 

 and the plane of motion makes with the axis of the objective an angle which 

 is the supplement of that of the tube-slide. At one end is a screw turned 

 by a watch-key, which acts as a stop to bring the objective always back to 

 the same position, and which also serves as a centering adjustment in the 

 direction of the slide, while the adjustment in the transverse direction is 

 effected by a similar screw working at right angles to the first. 



Objectives whose settings are approximately compensated for their focal 

 lengths can, by means of the clamp-screw on the objective-slide, be set once 

 for all in their proper position. Any number of objective-slides may be used 

 with one tube-slide. The two pieces fit one another accurately. The 

 objectives always return to the same position, so that the same part of 

 the object occupies the field of view. 



Hopkins, G. M. — Diminishing the power of an Objective. 



[" It is often desirable to dimiiilsh the magnifying power of an objective, and at 

 the same time increase its penetration. For example, if one possesses a 1^ in. or 

 2 in. objective, and desires to examine objects like minerals in the natural state, 

 crystals, seeds, &c., he will find it necessary to focus up and down upon the 

 object to see it in all parts. A 3 in. or 4 in. objective would furnish the desired 

 power, but it is not at hand. 



To increase the focal leugth, and at the same time enlarge the field and deepen 

 the focus, it is only necessary to place a double convex lens of, say, 5 in. focus 

 about half-way down the draw-tube. The action of such a lens is the reverse 

 of that of an amplifier."] 



i:ngl. Mech., XLV. (1887) pp. 310-1, from Scientific American. 



(3) Illuminating: and other Apparatus. 



Value of Achromatic Condensers.* — Mr, E. M. Nelson and Mr. G. C. 

 Karop write that an achromatic oil-immersion condenser has been made for 

 them by Mr. T. Powell (Mr. Nelson having, in 1882, suggested to him the 

 necessity for achromatizing the then chromatic oil-condenser) and that this has 

 enabled them to illuminate objects by solid axial cones of larger angle than 

 before ; the spherical aberrations of a chromatic condenser being so great 

 that only the rays passing through the centre or through a narrow zone of 

 the condenser could be focused on the object at one time. The result has 

 been a marked increase in resolution. In illustration of this increased 

 resolution they refer to a drawing of an areolation of the same valve of 

 Isihmia nervosa, which they figured in their former paper.f The straight 

 bars of silex, by which the central delicate perforated membrane was shown 

 to be attached to the margin of the areolation now have a trabecular 

 appearance ; the delicate membrane extends to the edge of the large 

 areolation, and has perforations more difficult to resolve than those in the 

 centre. 



They point out that this is not a correction of misinterpretation of 

 optical images, but a clear case of increased resolution, due to an improve- 

 m ;nt in optical appliances. Even now they do not wish to lay any claim 

 t ) finality, but to show that every advance in perfecting instrumental appli- 

 ances is attended by an increased gain in our knowledge of structure. In 

 addition to the new condenser they have used Professor Abbe's new 

 compensating eye-pieces, which give sharper images than those of the 

 Huyghenian construction. 



♦ Journ. Quek. Micr, Club, iii. (1887) pp. 41-3. 

 t Ibid., ii. (1886) pp. 269-71 (1 pi.). 



