ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 903 



one and the same eye-piece. After adjusting and viewing tlie image 

 in one field, remove the eye-piece and place it in the other tube ; if 

 the definition is then equally good the prism may be considered 

 satisfactory. Repeat the operation with the corresponding eye-piece 

 without altering the focal adjustment, and if the image is not equally 

 well defined, the eye-pieces do not match, that is to say their focal 

 powers differ. It is absolutely essential that each pair of eye-pieces 

 should be of equal power. Next ascertain whether the images in 

 both fields entirely coalesce when an object is viewed through the 

 tubes with both eyes. Place on the stage some round object large 

 enough to nearly fill the field of the eye-piece (a good Echinus spine 

 is generally sufficiently round and of the size required), adjust it in 

 the centre of the field of the vertical tube, so as to leave a concentric 

 ring of light around it, and then view the image in the oblique tube 

 with the same eye-piece as before ; should the image be equally in the 

 centre of the field, it is satisfactory. If, however, the image in this 

 tube appears a little out of centre towards the left of the observer, we 

 should not reject the instrument on that account, as in the opinion of 

 many experienced microscopists a slight lateral deviation of this kind 

 gives an increased stereoscopic effect to the image. But if the image 

 in the oblique tube be out of centre in the opposite direction (viz. 

 towards the right), the binocular arrangement is defective. Observa- 

 tions prolonged for even a short time will then cause great pain to the 

 eyes, and if continued would permanently injure even the strongest 

 eyesight. 



A reviewer of Mr. Swift's work * is sure that the above instruc- 

 tions " would lead the careful amateur to condemn 50 per cent, of the 

 binocular Microscopes issued by the opticians of this country." 



Bausch and Lomb Optical Company's Safety Nose-piece. — This 

 (fig. 164) consists of two tubes ; the upper A having the Society screw, 

 and fitting into the end of the body-tube ; and the -^iq i64 

 lower B, also with the Society screw, receiving the ^ 



objective. The lower tube is pushed out by a weak 

 spiral spring which is inside the upper one, but a 

 slight pressure is sufficient to press it in, and so 

 " prevent jamming of the objective into the object." 

 The lower tube is kept from rotating by the slot and 

 pin seen in the figure. 



Matthews' Device for Exchanging- Objectives.f 

 — ^Dr. J. Matthews, referring to the fact that the 

 joints of the stomach-pump fit together as cones and 

 that a joint was never known to give way, suggests, 

 as a most simple, inexpensive, and ready method for 

 exchanging objectives, a short adapter in the form of 

 a hollow cone which is screwed into the ordinary nose-piece, with 

 another piece screwing on the objective and coned down exactly to 



* Engl. Mech., xxxviii. (1883) p. 50. 



t Journ. Quek. Micr. Club., i. (1883) p. 305. 



