49G SUMMARY OF CURRENT RESEARCHES RELATING TO 



to bo nearly always inferred from the appearance of solid three-dimen- 

 sioned objects when seen under conditions which prevent them from 

 being examined in more than two dimensions at any one time. For it is 

 a long time before tho student can accustom himself, by focusing at suc- 

 cessive depths, and by making tho most of tho limited third dimensions 

 of depth which tho high powers of the Microscope provide, to judge 

 accurately of the forms of objects. And the novel conditions under 

 which a student sees with a Microscope effectually prevent him from 

 making the best of the impressions he receives. Thus, if the section of 

 a solid object presented tho appearance of a circle 1 inch in diameter, 

 and if two other sections at right angles to each other and to the first 

 section presented the appearance of a rectangular figure 3 feet by 1 inch, 

 nearly every one would readily infer that the shape was that of a cylinder 

 3 feet long by 1 inch in diameter. But precisely similar data when 

 presented in the field of the Microscope, do not readily lead the student 

 to any definite conclusions as to the forms of objects, and in reality a 

 long course of discipline is necessary in order to make him form any 

 clear conception of the actual shape of the object at which he is 

 looking. 



" I therefore think that it is expedient to begin the course of biologi- 

 cal teaching with organisms which only require the use of a Microscope 

 for the investigation of part of their structure, and thus to gradually 

 work downwards to the minutest organisms, in which the whole investi- 

 gation depends upon high microscopic powers. Thus the gradual 

 training in the use of the Microscope will proceed parallel with its 

 gradually increasing necessity." 



Cover-correction. — Herr C. Eeichert considers* that the "im- 

 portance of ' cover-correction ' by means of a screw collar is not so great 

 as it once was, because, in the first place, it is now possible to readily 

 obtain cover-glasses of a definite thickness, and, in the next place, 

 because all good Microscopes are now provided with a draw-tube. In 

 all high-class instruments, the draw-tube forms an important part, and 

 is less intended to increase the magnification than to correct for the 

 difference in the thickness of the cover-glasses. By means of varying 

 the length of the tube, we are able to produce an effect upon the image 

 similar to that which is the result of making the back lenses approach or 

 recede from the front lenses of the objective. The effect due to varying 

 the tube-length is noticeable in an objective such as No. 5, which has 

 a focal length of about 1/16 in., and is more marked as the power of 

 the objective increases. For example, if an objective having a focal 

 distance of about 1/10 in. be corrected for a cover-glass - 17 mm. thick, 

 when the tube is half drawn out, it may, by shortening the tube, be 

 made suitable for cover-glasses having a thickness of • 25 mm. to • 30 

 mm. ; and if the tube be fully drawn out, the objective will then be 

 suitable for cover-glasses from - 14 to 0*12 mm. 



" Those commencing microscopical studies should make themselves 

 familiar with the influence exerted by tho varying length of the tube, 

 and this may conveniently be done by studying a delicate test-object, 

 such as Pleurosigma angulatum, when the tube is extended or shortened 

 in the n anner already described." 



* Reichert, 0., ' Directions for using the Microscope,' translated by A. Frazer. 

 8vo, Edinburgh, 1887, 12 pp. (2 figs.). 



