288 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Burch's Perspective Microscope,* — Mr. G. J. Burch,in 1874, while trying 

 to devise means whereby the different j)laues of an object should be visible 

 under the Microscope without the adjustment of the focus to each, dis- 

 covered that, when two lenses are separated by a distance equal to the sum 

 of their focal lengths, the optical conditions are such that the magnitude of 

 the image bears a constant ratio to that of the object, no matter where upon 

 the optic axis it is situated — the ratio being that of the focal lengths of the 

 two lenses ; that a given displacement of the object along the axis causes a 

 displacement of the image in the same direction, but in the square of the 

 ratio. 



Further, that a picture drawn with the camera lucida under these 

 conditions has the perspective of an object magnified in the square of the 

 ratio, when it is brought within the proper distance of the eye. 



The field of view of the perspective Microscope is small, but may 

 be increased by using more than two lenses, and the author's researches 

 gave him reason to believe that, with glasses of wide angle specially 

 constructed, a high power, with sufiiciently large field, might be obtained. 

 Several uses other than microscopic were indicated, to which the instru- 

 ment can be applied. 



The paper, as read to the Koyal Society, was accompanied by diagrams, 

 showing, in two different ways, the changes of position of the principal 

 foci and principal points, &c., of a system of two lenses, as the distance 

 between them is varied, and a piece of moss was shown under the instru- 

 ment, in magnified perspective. 



Entomological Microscope.j — M. J, L.Weyers discusses the proper form, 

 &c., of a Microscope suitable for entomologists, which we read with atten- 

 tion until nearly its conclusion, without clearly apjn'eciating what the 

 author proposed in the way of an improvement upon the existing forms. 

 The last paragraph, however, dispensed with any necessity for again 

 reading the paper to supjily the missing clue. That jiaragraph runs as 

 follows : — " In tine, the compound entomological Microscope requires no 

 novel arrangement ; no unknown accessory. It simply aims at uniting in 

 one and the same instrument the different arrangements applied hitherto 

 separately to the usual compound Mcroscopes." 



Lehmann's Crystallization Microscopes.]: — Dr. 0. Lehmann has now 

 found it possible to construct a smaller, more portable, and cheaper form 

 of the Microscope, with which his observations on the growth of crystals 

 were made.§ 



The new instrument, as described by him under the name of the " Small 

 Crystallization Microscope," is shown in fig. 37, from which it will be seen 

 that it is not so much a Microscope of sj^ecial construction (in fact it is a 

 Merz 18G6 instrument) as an ordinary instrument adapted by the addition 

 of Cf^itain supplementary jmrts. 



The form of the stage is shown in fig. 38 ; the hollow rotating centre 6 

 carries the platinum covered stage a supported on a cylindrical ring 

 which is pierced with numerous holes to allow the products of combustion 

 to escape as indicated by the arrows, while to its lower side is fixed the 

 graduated circle c ; d is a handle by which the stage is turned, and which 

 abuts against a stop for the zero point of the scale ; the tube e in which 

 the stage rotates is centered by the four screws u ; the index is fixed to this 



* Nature, xxxv. (1887) p. 358. 



t L'oiiiptes IJeudus Soo. Entomol. Belg., 1886, pp. xc.-iii. 

 X Zeitschr. f. Iiistrumenteiik., vi. (1886) pp. 325-34 (3 figs.). 

 § ^'ee this Jcuriial, I8}<5, p. 117, 



