456 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MICROSCOPY. 



a. Instruments, Accessories, &c.* 

 (1) Stands. 



Burcli's Perspective Microscope. t — In 1874, Mr. G. J. Burcli " dis- 

 covered, a form of Microscope giving constant magnification along the 

 optic axis, so that the objects were shown by it in microscopic perspective," 



By writing (/^ -{-/^ -f- H) for the distance between two thin lenses, he 

 obtained for the formula of the system 



/,(/,4-H> -/,/,(/, +/, + H) _ 



u being the distance from the object to the first lens, and r that from the 

 second lens to the image. 



Putting H = in this equation, three things result : — 



1. du I dv, which represent the longitudinal magnification, becomes 

 constant, namely — (/, / fi)- ; 



2. The lateral or angular magnification, /g //i, is also constant ; 



3. A picture of an object so magnified, drawn with the camera lucida, 

 when viewed from a distance fo j /i times less than that at which it was 

 drawn, has the perspective belonging to an object magnified ( f.^ j f^f times. 



The distance at which the eye must be placed is great, but may be 

 reduced by emjiloying three lenses, the distance between the first and 

 second being ( ^'i -4-/2 +/''/''05 ^^*^ ^^^^ between the second and third 

 {fz+fz + raf.^- 



If the lenses are nearly but not quite in the afocal position, greater power 

 and a wider field may be obtained ; but it is at the ex2:)ense of the jienetra- 

 tion, which may, however, with advantage be limited to the thickness of the 

 object. The instrument ofters great advantages for artistic purjioses, but 

 lenses or mirrors of sjiecially wide angle are needed for the farther develop- 

 ment of the invention. 



The optical conditions of a system of two thin lenses at varying 

 distance apart are shown by diagrams. 



In diagram 1 the u and v of the f(n"mula employed are set oflf as 

 abscissEe and ordinates, and the curves (which are rectangular hyperbolas) 

 drawn for several values of H. In the afocal position of the lenses, the 

 curve degi'ades into a line which is a tangent to all the hyperbolas at the 

 point {fi,fo)- The locus of vertices and locus of centres of these curves 

 being straight lines, and the hyperbolas all touching the point (/ij/a), it is 

 shown that the principal feci, principal points, and equivalent focal length 

 for any given position of the lenses, can be found by rule and compasses, 

 without drawing the curve. 



In diagram 2 the actual position of the lenses, their principal foci, 

 separate and combined, and the principal points, positive and negative 

 (answering to the vertices of the curves in diagi-am 1), are plotted down 

 a.s abscissae, the values of H on an enlarged scale being taken as ordinates. 



Diagram 3 shows the same for two lenses of equal local length. 



Comparison of these two diagrams suggests the employment of the 

 term " pseudo-principal points " for those positions at which the magnitude 



* This subdivision contains (1) Stands; (2) Eye-pieces and Objectives; (3) Ulunii- 

 nating and other Apparatus ; (4) Photo-micrography ; (.5) Microscopical Optics and 

 Manipulation ; (C) Miscellaneous. 



t Froc. Koy. Soc., xlii. (1887) pp. 49-50. Sec also this Journal, ante, p. 288. 



