ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1073 



taken, but it was not the image for which I was in search. Examples 

 of this sort arc among the prints which I will exhibit to the Society. 

 I design to add to my experiments on the subject, the examination 

 of the effect of changing the focus of the focusing glass to corre- 

 spond with the difference between the visual image of a diatom, 

 showing little dots or areolfe and that which shows dark ones. 

 Everybody has noticed that a slight change of focus with a bigh 

 power produces this change of appearance, and if the focusing glass 

 were adjusted for the image which is complementary to the one 

 desired, and then the focusing done in the usual way, the result 

 might be that which is sought. It has at least seemed worth the 

 experiment, but a press of other work has prevented my making a 

 satisfactory test of it before the time of our meeting." 



Images in the Binocular Microscope.* — Mr. E. M. Nelson writes, 

 " Binoculars give less critical pictures than monoculars, for the very 

 good reason that half an objective will not perform so well as a wliole 

 one. All prisms are defective ; therefore the image in the left tube 

 is worse than that in the right. The image in the binocular, there- 

 fore, consists of an indifferent picture in the right-hand tube, and a 

 worse one in the left. Observers put up with this for the sake of 

 the stereoscopic effect, which is gained at the cost of a critical 

 image. . . . 



Opticians know very well that the eye will accommodate itself, 

 and combine almost anything ; therefore little or no pains are taken 

 to send out binoculars in perfect adjustment. I will mention a fault 

 wliich is frequently seen in binoculars exhibited at the Societies. 



1. The axis of the left-hand tube does not make the proper in- 

 clination with the other ; this causes the field of the left-hand tube 

 not to coincide laterally with that of the right hand (fig. 243). 



2. If the axes of the eye-pieces are not in the same plane, the field 

 of one tube will be either above or below the other (fig. 244). 



Fig. 245 shows what is often found, viz. Nos. 1 and 2 combined. 



3. The focus of each tube should be carefully adjusted, either by 

 the tube or by the eye-piece. I have my own done by collars round 

 the eye-pieces ; but the tube-length method is preferred by some, and 

 is just as efficient. 



4. The eye-pieces should be matched in power. 



5. The position of the prism in its carrier should bo correctly 

 adjusted. One would tliink that a very trifling movement in the 

 prism would make a very great difference in the position of the imago 

 of the field, but such is not the case. The plane of the base of the 

 prism should bo at right angles to tlic axis of the objective ; if, how- 

 ever, this is tilted through an angle of 20'', one will be surprised at 

 the small diflcrence it makes. Any twist in the prism would make a 

 very serious fault ; in otlier words, the planes of the reflecting sur- 

 faces of the prism must be at right angles to the plane of the axed 

 of the tubes. Sec fig. 24*5, which shows that when the prism is out 

 of adjustment an object will not occupy the same position in each 



♦ Engl. Modi., xlii. fl885) p. 202 (0 fi^s.). 



