IO6 VISION WITH THE COMPOUND MICROSCOPE 



for securing binocular vision with the highest powers. We have used 

 the latter of these with perfect satisfaction, but all that is required 

 is at the disposal of the student in the arrangement of Powell and 

 Lealand. 



To those who have used these forms of binocular habitually it 

 has been a frequent source of surprise and perplexity that, although 

 theoretically such a form as that of Powell and Lealand's is non- 

 stereoscopic, yet objects studied with high powers have appeared as 

 if in relief, and the effect upon the mind of stereoscopic vision has 

 been distinctly manifest. The Editor was conscious 

 of this for many years in the use of the Powell and 

 Lealand form, with even the ^V^h of an inch power 

 of the achromatic construction .; at the time he inter- 

 preted it as a conceptual effect ; but it always arose 

 when the pupils fell upon the outer halves of the 

 Ramsden circles. The explanation, Dr. A. C. 

 Mercer considers, 1 is due to Abbe. Since (fig. 85) 

 when the eye-pieces are at such a distance apart that 

 the Ramsden circles correspond exactly with the 

 pupils of the eyes, centre to centre, the object appears 

 flat. But if the eye-pieces be racked down, so as 

 FIG. 85. to be nearer together, the centres of the pupils fall 



upon the outer halves of the Ramsden circles and we 

 have the conditions of orthoscopic effect ; while if they be racked up 

 so as to be more separated, the centres of the pupils fall on the inner 

 halves, and we have pseudoscopic effect. 



The Optical Investigations of Gauss. Before leaving this section 

 of our subject, in which we have endeavoured, with as much clear- 

 ness as we could command, to enable the general reader to com- 

 prehend with intelligibility the principles of theoretical and applied 

 optics as they relate to the microscope, we believe we shall serve 

 the higher interests of microscopy, and the wants or desires of the 

 more advanced microscopical experts, if we endeavour to present in 

 a form either devoid of technicality or with inevitable technicalities 

 explained a general outline and then an application of the famous 

 dioptric investigations of Gauss, an eminent German mathematician, 

 who, amongst many bther brilliant labours in applied mathematics, 

 expounded the laws of the refraction of light in the case of a co-axil 

 system of spherical surfaces, having media of various refractive in- 

 dices lying between them. 



Although the assumptions upon which the formula? of Gauss 

 rest are not coincident with the conditions presented by the lens- 

 combinations which are employed in the construction of modern 

 objectives of great aperture, the results, nevertheless, furnish an 

 admirable presentation of the path of the rays and the positions of 

 cardinal points, even in the microscope as we know and use it. 



We remember that the microscope is largely used in England 

 and America by men who can only employ it in their more or less 

 brief recessions from professional and commercial pursuits, but who 

 often employ it with enthusiasm and intelligent purpose. Much 



1 Journ. H.M.S. ser. ii. vol. ii. p. 271. 



