MICROSCOPY. 



507 



during the last few years, only two have come 

 into considerable use. The first of these, by 

 Mr. J. W. Stephenson, of London, is a reintro- 

 duction of a pair of reflecting prisms of the form 

 adopted by Prof. J. L. Riddell, of New Orleans, 

 the original inventor of the binocular micro- 

 scope. These prisms, shown in Fig. 14, located 

 close to the back lens 

 of the objective, di- 

 vide the pencil of 

 light into two sym- 

 metrical halves, which 

 are turned obliquely 

 forward, the image 

 being at the same 

 time erected by a 

 larger prism placed 

 above at d. Being an 

 erecting arrangement, 

 and having the oculars 

 inclined at the most 

 convenient angle for 

 vision when the stage 

 is horizontal, this con- 

 trivance is well adapt- 

 ed, and it has proved 

 satisfactory, for a dis- 

 secting or preparing 

 microscope. A slight 

 loss of definition in 

 both fields, and the 

 impossibility of using 

 the same body as a 

 monocular, has limit- 

 ed it to a narrower 

 range of usefulness 

 $han that of the Wen- 

 ham form. On the other hand, Prof. Abbe's 

 binocular, introduced in 1880, is a removable 

 ocular which is applicable to any stand. In 

 it the beam of light is not laterally bisected, 

 but enters a pair of prisms, a, ft (Fig. 15), a 

 portion of the whole pencil passing directly 

 through to one ocular, at B, while the other 

 portion is reflected from the surfaces between 

 the prisms a and ft, and from the second sur- 

 face of the prism V to the other eye, at B'. 

 The intensity of the axial and reflected pencils 

 is unequal, the image in the direct tube being 

 much superior to the other, as in the Wenham 

 form. It is subject, however, to the disadvan- 

 tage, as compared with that, of such slight loss 

 of definition as may be caused by the passing 

 of the direct rays through the prisms ft, a. The 

 difference in size of the images, caused by the 

 longer course of the reflected rays, is corrected 

 by a corresponding difference in the construc- 

 tion of the oculars. The arrangement is not 

 essentially stereoscopic, and may be used for 

 the sake of the comfort of using both eyes at 

 once, with the full capacity of both tubes, and 

 with similar images in both eyes. A stereo- 

 scopic effect, however, is produced at will by 

 placing over the oculars caps which obstruct 

 the inner half of each tube, so that only the 

 outer half of each pencil is utilized. The open- 



ing at B, in the figure, is concentric to the 

 optical axis, as for non-stereoscopic vision, 

 while that at B' is shown with only the outer 



FIQ. 14. KIDDELL-STEPHEN- 

 SON BINOCULAR. 



FIG. 15. ABBE'S BINOCULAR OCTJLAB. 



half in use, as for stereoscopic vision. A sliding 

 adjustment for the distance between the two 

 eyes is controlled by a milled head at D. This 

 binocular is well adapted to the small Conti- 

 nental stands, being adjusted for a tube-length 

 of 13-16 centimetres. With an adapter insert- 

 ed in place of the lengthening draw-tube of the 

 stand, it is equally applicable to short instru- 

 ments of the American type It is not made of 

 a size well adapted to large instruments with 

 long tubes, as it would then become heavy and 

 clumsy by reason of the large prisms required. 

 When the monocular stand is used, an eye- 

 shade, so located as to prevent the formation 

 of images in the open but unemployed eye, is 

 of material advantage in lessening fatigue both 

 of eye and brain. The clumsy devices for- 

 merly made for this purpose were soon aban- 

 doned. But lately practical and really useful 

 substitutes have been devised by Dr. L. B. 



FIG. 16. EYE-SHADE. 



Hall, Mr. E. Pennock, and others. The last 

 published form, devised by the writer and 

 shown in Fig. 16 ("A. M. M. J." 1884, p. 83), is 

 a screen of hard rubber or blackened metal 

 about 1-J- inch in diameter, with an extension 

 in the form of a band crossing in front of the 

 nose and encircling the top of the tube just be- 

 low the ocular. Being attached to the body, 

 this requires no new adjustment with each 

 change of ocular. It is easily reversible, which 

 often becomes desirable on account of the 



