"joumai,*J'eb°i'i87a'] ^^ the Micvoscope. 97 



construction (a double eye-piece with four lenses, similar to those 

 of the terrestrial spy-glasses) the magnifying power of the instru- 

 ment, and also to nearly the same degree the penetration, is raised, 

 by a tube of 430™"^, 20, 28, 55, 97, and 137 per cent, (the latter, 

 of course, with diminution of the field), more than the same objec- 

 tive (Hartnack's, No. 7) and eye-piece (No. 3) with a tube 200'"" 

 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 15 to 20™™ in diameter, and with a diaphragm between, 

 having an aperture of fi'om 8 to 9"™. For the two inferior 

 lenses, a common Huyghen's eye-piece would be the best. Such 

 a combined eye-piece, with a tube 420'"™ long, would raise the 

 power of the instrument 97 per cent. The use of an achromatic 

 condenser adapted for obUque 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 1™™ distance will 

 give the first real image at a distance of 200™™ from the second 

 chief point of the objective; and combined with an eye-piece in 

 Listing's manner, having a power of 25 diameters by itself, and a 

 tube 450™™ long, the magnifying power of the whole instrument 

 would be 5000 diameters. 



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

 dinal points are in the same order as in a cancave lens, and the 

 focal distance of the whole microscope (not of the objective) would 

 be equal to — '5™™, with a magnifying power of 400 diameters 

 for a visual distance of 200™™. 



In the Listing instrument the order of the cardinal points 

 would be inverted and analogous to a convex lens, with a focal 

 distance of the whole microscope equal to -)- • 04™™, with a magni- 

 fying power of 5000 diameters. In the first case the objective 

 would have a focal distance of 3™™ ; m the last of 1™™. The difi'er- 

 ence between the two chief points of the whole microscope is in both 

 cases nearly equal to the whole length of the tube. In the last 

 arrangement the whole microscope is analogous to a convex lens 

 with very short focal distance. 



In a second paper Professor Listing gives further facts con- 

 cerning this arrangement. An objective with a focal distance 

 of 1™™ has the first image 201™™ distant from the second 

 chief point. The first magnifying is = 200. The middle eye- 

 piece of two achromatic lenses with 25™™ focal distance, and 15™™ 



VOL. III. H 



