ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 611 



Although the apparatus in this form may be thought to fulfil all 

 requirements, Dr. Sohncke considered it esi^ecially necessary to add 

 an additional contrivance for indicating, without further measure- 

 ment, the most characteristic phenomenon in the position of Newton's 

 rings, viz. that those ring-j)oints which are in the plane of incidence 

 passing through the centre of the rings, all lie in a straight line 

 which rises obliquely towards the light. The greatest inclination of 

 this " fundamental line " towards the horizon is 19° 28'. If we have 

 an arrangement by which the Microscope with any angle of incidence 

 can be given a movement parallel to the " fundamental line," then 

 when any one ring (in the central plane of incidence) is clearly seen 

 by proper focusing of the Microscope, all the rings in succession will 

 also be clearly seen by the movement in question ; whilst if the Micro- 

 scope were moved horizontally, they would very soon be out of focus. 

 This requirement is carried out in the present instrument by the 

 guides of the lower slide being fixed, not upon the horizontal base G, 

 but upon the plate P, which is movable on an axis at right angles to 

 the plane of incidence, and can be fixed at any required inclination 

 between 0° and 20°. That the object may not be disturbed by the 

 inclination of the plate, it is cut out somewhat in the shape of a 

 horse-shoe. To use this arrangement the plate P must be placed at 

 the angle w of the " fundamental line " for the particular angle of 

 incidence 6. The value of oo is obtained from the formula : — 



sin 6 . cos 6 



The Microscope is then to be placed at the required angle of 

 incidence. In order to do this direct, a plumb line, instead of an 

 index, is used for reading off the " angle " on the quadrant, as an 

 index would join in the inclination of the plane P. The lower 

 slide has now only to be moved parallel to the plane of incidence, by 

 means of the screw M2, in order to see all the rings pass across the 

 field in complete distinctness. 



Dr. J. H. L. Flogel describes* a method of determining the thick- 

 ness of diatoms by the examination of the Newton rings formed when 

 they are illuminated by reflected light from a Lieberkiihn. It con- 

 sists simply in tilting the slide at an angle, the light being admitted 

 to the Lieberkiihn through a small excentric aperture in the dia- 

 phragm, reaching the objective only after reflection from the prepara- 

 tion. 



Harris & Son's Portable Microscope. — This (figs. 93 and 94) is 

 a somewhat ancient form, probably fifty years old, but is arranged 

 on an ingenious plan to secure portability. When set up for use it 

 takes the form shown in fig. 93. By unscrewing the tube, and 

 screwing it into the lower side of tlic ring which holds it, and closing 

 the tripod legs together, it is reduced to the form shown in fig. 94. 



* Aicb. f. Mikr. Anat., vi. (1870) pp. 472-514. 



