•<9 



ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 127 



it of a diaphragm having a spiral opening (as figured) will provide 



a pencil of light at varying degrees of obliquity throughout the range 



of aperture of the condenser. The azimuthal direction of the incident 



pencil will of course be controlled either by rotating the object or the 



condenser carrying the diaphragms ; 



whilst the rotation of the spiral on Fig. 19. 



the fixed slot will not change the 



direction in azimuth, but only in 



altitude, so far as the aperture of the 



condenser will permit. 



The diaphragms hitherto applied 

 for this purpose have for the most 

 part had but a very limited range of 

 action in varying the angle of oblique 

 incidence of the illuminating pencil ; 

 indeed, with the exception of Professor Abbe's traversing diaphragm 

 plate fitted to his recent forms of substage condensers, we are not aware 

 that any efficient means for this purpose has been devised. Professor 

 Abbe's plan requires a special horizontal rack and pinion made to the 

 substage to allow the free motion of the diaphragm. 



High Amplifications. — Eeferences have been made by a lady 

 correspondent in recent numbers of the ' American Journal of Micro- 

 scopy,' to an amplification of upwards of 20,000 diameters obtained 

 with a ^ objective using an eye-piece of ^L equivalent focus. On the 

 usual assumption that a 1-inch objective gives a magnification of 

 10 linear, a i would give 50, and a gL eye-piece 500 : the combined 

 amplification would therefore be 60 x 500 = 30,000 linear. A few 

 instances of practical results obtained with high amplifications cannot 

 but make us feel sceptical of the value of the definition obtained with 

 a 5V eye-piece. 



Sir John Herschel, in his ' Treatise on Light,'* mentions having 

 "viewed an object without utter indistinctness thvongh a Microscope by 

 Amici, magnifying upwards of 3000 times in linear measure." 



In the very early days of the collodion process in photography, 

 Mr. Wenham exhibited a micro-photograph of P. angulatum magnified 

 15,000 linear : it is no disparagement to the photograph (a copy of 

 which we have recently inspected), to say that it is not remarkable for 

 distinctness. It was produced with the first compound achromatic -^^-^ 

 ever made in this country ; and the lens coming from the hands of an 

 amateur optician, and the photograph being produced by the same 

 amateur photographer, the result was then regarded as an almost 

 marvellous specimen of practical skill and ingenuity. 



Some thirteen or fourteen years ago, Hartnack had a micro-photo- 

 graphic transparency on ground glass of P. angulatum exhibited in his 

 atelier in the Place Dauphine, Paris. The magnification was about 

 3000 linear; the image was well defined to the edges — about 16 inches 

 square — and was produced with one of his then best No. 10 immer- 

 sions. 



* Encyc. Metrop., p. 581. 



