ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 829 



the definition also of colourless objects is much improved. When, 

 however, the prism is turned round and the light taken by reflection 

 from the coloured glass, a delicate monochromatic light is obtained — 

 very grateful to the eye — and many objects with low powers (1-inch 

 and 2-inch) are brought out with a clearness and beauty that must be 

 seen to be appreciated. Monochromatic light has been used with 

 much success by many observers, and this prism is a very convenient 

 mode of getting it. 



With the eye-piece and prism in combination I can show 

 P. angulatum and quadratum with splendid eifect, magnified 1100 to 

 1500 diameters, and then by simply refixing the eye-piece (as in 

 Fig. 190), and with the monochromatic light, I can show whole 

 insects of small size with a 3-inch objective, the same illuminators 

 serving thus two very widely different ends. 



It will be seen that I invert the fixed lens as in the drawing to 

 save using an inconvenient length of tubing. I think, however, the 

 same purpose might perhaps be better answered „ „„ 



by mounting the field-lens in a very short piece 

 of tube as in Fig. 192 E, furnished with two screw 

 heads, and running it up and down by means 

 of two parallel slits in the tube C. In the same 

 way a small diaphragm might perhaps be used with advantage between 

 the two lenses of the eye-piece. I have not obtained any advantage by 

 the gradual approximation of the two lenses of the Kelner eye-piece ; 

 they act best for each purpose (high or low power illumination) either 

 close together in their normal position or a good distance apart. 

 This arrangement of the prism cannot be used by daylight without 

 giving monochromatic light ; however it might be remedied by not 

 cementing the piece of blue glass to the prism. In that case for night- 

 work either a piece of deeper tint would have to be substituted when 

 the monochromatic light was required, or two pieces of the same 

 pale tint must be used." 



Difference in tte Appreciation of the apparent Size of Micro- 

 scopical Images by different Observers.* — M. C. Montigny has 

 undertaken an investigation of this subject with the object more 

 especially of determining whether the differences which are well- 

 known to exist in different persons, depend entirely upon the inequa- 

 lities in the distances of distinct vision, objects, as is well known, 

 appearing to be larger to short-sighted than to long-sighted persons. 



In the first series of experiments the observers were not accus- 

 tomed to microscopical observations, being nine pupils of the upper 

 class of the Brussels Athenee, and the objects were discoid globules 

 of human blood, which were compared with twelve small circles of 

 different sizes, 0*5 mm. to 7 mm., traced on a card, and elliptical 

 globules of frog's blood, comj)ared with ten ellipses, the long axes of 

 which increased from 1 mm. to 10 mm. The cards were placed at 

 very nearly the distance of distinct vision of the particular observer, 

 previously carefully ascertained. The same globule was always 



* Bull. Acad. K. Sci. Belg., xlix. (1880) pp. 670-8. 

 Ser. 2.— Vol. I. 3 I 



