106 VISION WITH THE m.AI POUND MICROSCOPE 



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

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

 is .-.t th.- disposal of the student in the arrangement of Powell and 



Lealand. 



To those who have used these forms of binocular habitually it 

 has li.-en a frequent source < .f surprise and perplexity that, Although 

 tlieor.-tically such a form as that of Powell and Lealand's is noii- 

 stereoscopic, vel objects studied with high powers have appeared as 

 jfjn relief, ind tin- effect upon The mind of stereoscopic vision has 

 been distinctly manifest. The Editor was conscious 

 J _ of tliis for many years in the use of the Powell and 



Lealand form, with even the ^Vth 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) 

 \\ hen 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 

 tlat. Hut if the eye-pieces lie racked down, so as 

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



upon the milcr 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 //" J>I-!H<-!/>/I>K 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 <i </< n< rnl nut/in,- <i,nl then <m application of the famous 

 dioptric investigations of (,'nn.^, an eminent (lermaii mathematician, 

 who, amongst many other brilliant labours in applied mathematics, 

 expounded the IHH-H ,,/' //,, /,//-,/, /;,,// ,,/' //',//// in tin' case of a co-axial 



ni/stem i>/ x/Jit'i-ii-H/ xiirf<-<>n, Imrnnj ni<><H<i of rur'ninx i-<f /</<! i re in- 



s li/iiKJ I" In; ' n lln-ui . 



Although the assumptions upon which the formula' of (Jauss 

 are Qol rnincident \\ith the conditions presented by the leiis- 

 combinations \\hich are employed in the construction of modern 

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

 adinii-al)le presentation ,,)' the path of the rays and the positions of 



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



\\ < remember that the microscope is largely used in England 



""I America b\ men \\hocaii only einplov it in their more or less 



I recessions from professional and commercial pursuits, but who 



ten employ ujth enthusiasm and intelligent purpose. .Much 



< i . ii. \ ill. ji. p. -271. 



