ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1059 



It consists of a brass angle -plate with slots, wbicli slide on suitably 

 arrangcil milled-bead screws beneath the stage. When the plate is iu 

 position the screws are tightened, and it is firmly clamped to the stage, 

 forming a continuation on either or both sides at the same level. 



Riddell's Binocular Compound Microscope. — Prof. J. L. Riddell, 

 of New Orleans, Louisiana, was the original inventor of the Bino- 

 cular Compound Microscope witli one objective. A description of his 

 form of prisms was published in 1854,* but the instrument itself has 

 not been figured complete, either here or abroad. Prof. Riddcll's own 

 instrument is the property of the United States Government, but by 

 the courtesy of the Surgeon-Genoral of the United States Army 

 (acting through Dr. John S. Billings, Curator of the Army Medical 

 Museum, Washington) it was placed in the hands of Mr. J. Grunow, 

 of New York (brother of the original constructor), by whom a dupli- 

 cate was made and sent to this country, and is reproduced in fig. 234. 

 The arrangement of the binocular prisms is shown in section in 

 fig. 23.5, as drawn iu the original paper. 



The pencil of rays emerging from the objective I is divided in two, 

 each half passing respectively into the right and left prisms. The 

 path of the rays is a, h, e, d (the object is at o). In the prisms 

 figured Prof. Riddell remarked that the equal angles at the long face 

 are 45", consequently the rays suifer a slight chromatic dispersion at 

 c, but he found no attendant practical disadvantage, unless eye-pieces 

 of unusually high power were used. By making the equal angles of 

 the prisms 85^ or 86°, so that the immergence and emergence would 

 be at right angles to the glass planes, the dispersion would be avoided ; 

 but then another difficulty would arise by the transmission of direct 

 rays (without reflection from the binocular prisms) from the object, 

 which would destroy the binoc"ular image. 



To facilitate the perfect coalescence of the images in the field of 

 view for every width of eyes. Prof. Riddell provided (1) a means of 

 regulating the inclination of the prisms by mounting them in hinged 

 frames, so that while their lower terminal edges remain always in 

 parallel contact the inclination of the internal reflecting faces can be 

 varied by the action of a milled head in front of the prism box ; 

 (2) the lower ends of the b'nocular tubes are connected by travelling 

 sockets, moving on one and the same axis on which are cut corre- 

 sponding right- and left-handed screws, so that the width of the tubes 

 may correspond with that of the prisms; and (3) the upper ends of 

 the tubes are connected by racks, one acting above and the other 

 below the same pinion, so that right- and left-handed movements are 

 ajnirnunif;ated by turning the pinion. 



Prof. Riddell found that in many cases it was advantageous to 

 employ two small concave mirrors rather than one largo one, so as to 

 equalize the illumination in both fields. 



To obviate the inconvenience of using tho instrument always in 

 the vertical position, small rectangular equilateral prisms are so 

 mounted iu brass caps as to bo slipped at pleasure over the eye-pieces. 



♦ Quart. Journ. Micr. Sci., ii. (1851) pp. 18-24 (t figs.). 



