1869.] 165 IHagen. 



In all microscopes the dioptric arrangement is now analogous 

 to the astronomic spy-glass ; they have but one real image, from 

 which the virtual image is formed and brought to the eye of the 

 observer. 



Professor Listing proposes to have two real images, and in this 

 way to form three successive augmentations instead of two, as be- 

 fore. It is well known that by a prolongation of the draw tube, or 

 by increasing the distance between the objective and the eye-piece, 

 the image becomes successively greater, but the definition and pene- 

 tration is by no means better. Professor Listing has made some ex- 

 periments, and states that with an eye-piece of his construction (a 

 double eye-piece with four lenses, similar to those of the terrrestrial 

 spy-glasses) the magnifying power of the instrument, and also to 

 nearly the same degree the penetration, is raised, by a tube of four 

 hundred and twenty mill., 20, 28, 55, 97 and 137 per cent, (the lat- 

 ter, of course, with diminution of the field), more than the same 

 objective (Hartnack's, No. 7) and eye-piece (No. 3) with a tube two 

 hundred mill, in length. The object was Pleurosigma angulatum, and 

 Professor Listing assures us that the latent power of the objective is 

 developed by this means in an astonishing manner. He also remarked 

 that the so called Erectors have long been used, but always with a 

 low power and a short tube. The most advantageous form for the 

 eye-piece would be, for the two superior glasses, achromatic lenses 

 from fifteen to twenty mill, in diameter, and with a diaphragm be- 

 tween, having an aperture of from eight to nine mill. For the two 

 inferior lenses, a common Huyghcn's eye-piece would be the best. 

 Such a combined eye-piece, with a tube four hundred and twenty 

 mill, long, would raise the power of the instrument ninety seven per 

 cent. The use of an achromatic condenser adapted for oblique 

 illumination is necessary for high powers. The experiment was only 

 successfully made with the best objectives of English artists, or with 

 the excellent new Hartnack objectives. 



According to his calculation, an objective of one mill, distance will 

 give the first real image at a distance of two hundred mill, from the 

 second chief point of the objective, and combined with an eye-piece 

 in Listing's manner, having a power of twenty-five diameters by itself, 

 and a tube four hundred and fifty mill, long, the magnifying power 

 of the whole instrument would be five thousand diameters. 



In the common arrangement of the microscope, the dioptric cardi- 

 nal points are in the same order as in a concave lens, and the focal 



