( 290 ) 

 NOTES AND MEMOEANDA. 



Tolles' Immersion ^th.— In a letter to Mr. Slack, Dr. Woodward 

 says: — "The Tolles' immersion ^^tli, by wliicli the Amphipleura pictures 

 were made, works either dry or wet, the compensation being effected 

 simply by altering the distance of the front lens from the other two, 

 by means of the screw collar. There is also a low-angle extra front 

 for ordinary work. 



" I find with the high-angle fronts the following measurements : — 



Magnifies at 4 ft. focus ; 

 Micrometer screen and 

 Angle. without eye-piece. 



Dry: uncovered, 110° 225 diam. 



„ for thickest cover, 140° 250 „ 



Immersion : uncovered, 140° 250 „ 



„ for thickest cover, 170° upwards . . 275 „ 



" With central light, and on Podura, or anatomical objects, I find 

 this objective admirable. 



" I wish I could speak as favourably of Mr. Tolles' higher powers. 

 They are very good indeed, but I have yet to see one of them which will 

 rival the so-called -i^g^th immersion of Powell and Lealand." 



American Microscopical Apparatus. — The following brief account 

 of the apparatus at the last American Association is given in the 

 September No. of the ' American Naturalist ' : — Among the novelties 

 may be noticed the observation of the electric induction spark by the 

 micro-spectroscope, by Prof. Vander Weyde; the oblique illumi- 

 nation of transparent objects under high powers by means of light 

 reflected from a plane mirror lying upon the stage and directly 

 beneath the mounted object — a little expedient of great practical 

 convenience, also by Prof. Vander Weyde ; the adoption of the 

 Wonham Binocular arrangement by Zentmayer ; and the somewhat 

 general introduction into use of the eye-piece condensers with a wide 

 horizontal illumination (for binoculars) upon the plan proposed by 

 Prof. Ward at the Troy meeting last summer. Mr. Bicknell places 

 the stop-plate between the lenses of the condenser, instead of below 

 them ; and Prof. Ward, while retaining the eye-piece arrangement for 

 use with low powers, for high powers combines the centring adjust- 

 ment, Iris diaphragm, and stop-plate, with an achromatic combination 

 of larger angle and more perfect corrections. The committee on 

 uniform standards in the powers of objectives and eye-pieces being 

 unprepared to report, Messrs. Ward and Bicknell reported verbally, 

 and the committee was continued until the next meeting. While an 

 exact uniformity in the amplifying power of lenses in the same 

 denomination is not to be looked for, it is believed that much of the 

 existing confusion may be remedied. Many microscopists, the 

 speakers among the number, have long been accustomed to alter the 

 denomination of their lenses so as to represent, as nearly as prac- 

 ticable, their amplifying power when in actual use ; and probably the 

 principal makers in this country will freely co-operate with micro- 



