ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 507" 



mining the refractive index, and they eliminate the bifocal effects 

 described by Sorby.'"' If they are employed with the Schroder van der 

 Kolk method, the finger or other screen must be introduced along the 

 direction of the length of the slit. 



If the slit is employed in the Beck method, it must be parallel to the 

 boundary along which the effects are to be obtained. The result is that 

 it is only occasionally that the principles described above can be applied. 

 The slit has, however, in this case another important recommendation. 

 When parallel to the boundary of the minerals to be compared, it gives 

 as definite results as a circular aperture with a diameter equal to its 

 width, and at the same time the field is much better illuminated. 



Dr. Tutton pointed out at the reading of the paper that the slit could 

 also be employed to obtain monochromatic light for microscope studies 

 of crystals, if the spectrum from a prism were thrown upon it in such a 

 manner that the individual colours were parallel to the slit and the 

 length of the spectrum at right angles to it. 



(5) Microscopical Optics and Manipulation. 



Improved Immersion Oil.f — Flatters and Garnott have introduced 

 this oil to replace the thickened cedar-wood oil, and it answers the 

 purpose of an immersion oil in microscopical examinations. Its refractive 

 index is I'SIO at 30° C. ; it keeps clear even in the cold, is easily 

 removed from the slide, and does not readily dry. It shows a neutral 

 reaction when tested with litmus-paper. 



Orthostereoscopic Imag-e. J — E. M. Nelson writes as follows : — 

 " With reference to ' N. E. B.'s ' remarks on the value and beauty of 

 a good orthostereoscopic image in a microscope, every real lover of that 

 instrument will agree ; microsoopists will also assent to his praise of the 

 Wenham form of instrument, although some may not be prepared to 

 follow his advocacy of its employment with the highest powers. The 

 position of the Wenham seems to be that it is a very good form of 

 instrument. Its best effects are obtained with objectives from 1 in. 

 to 4 • 10 in., a range of the most useful powers ; but on either side of this 

 there is a falling off. Nevertheless, it has an advantage not possessed by 

 all the other forms, viz. that it can be instantly converted to a monocular ; 

 there is further the advantage of its simplicity of construction by a single 

 prism. The sole reason why it has been so universally condemned by 

 the professional microscopists in this country is that it is not used on 

 the Continent ! The reason why it is not adopted on the Continent is 

 simply that ii cannot be applied to the short-tube microscope. There is, 

 as ' N. E. B,' points out, not the slightest justification in the state- 

 ment that a Wenham cannot be advantageously used with apertures 

 greater than 0-34 N.A, 



" Here are the results of experiments made with three kinds of 

 binoculars, carefully adjusted to the same magnifying powers. The 



* H. C. Sorby, :\Iineralogical Mag., i. (1877) pp. 97 ; xv. (1909) p. 189. 



t Lancet, Sept. 1, 1917, p. 350. 



X English Mechanic, cvi. (1917) pp. 17, 4.1. 



