280 Transactions of the [ l^^^^ii; J'JSeTJs'Ta ' 



In one of the plaotographs exhibited to the meeting there is a 

 somewhat diiferent aiTangement. The body is twelve inches long, and 

 all parts are more substantial ; the radius bars are an inch wide and 

 an eighth of an inch thick, jointed by a broad hinge, and further 

 steadied by strong circular slides ground to the edge of the back 

 plate ; they are opened to the width of the eyes by two racks and a 

 pinion operated by a large mill at the back. The coincidence of 

 Horopter and Focus is secured by a wheel and pinion, working a 

 peculiar parallel motion giving action to the firm slides to which the 

 cradle of the body is fixed, and so contrived as to act in any position 

 of the radius bars. 



The motion of the stage is great enough to admit the thickest live- 

 box, and a range of powers from 2 inches to one-quarter. 



A cradle-joint admits of the body assuming any angular position. 

 It is movable around a vertical centre, and the whole stands on a mas- 

 sive circular foot. 



In a second instrument there is a greater range of adjustment for 

 Horopter is obtained by making the body-cradle rigid, and sliding the 

 body in it as required, two mills holding it in any position. It is much 

 more compact and portable, and works with a shorter body, has a me- 

 chanical stage, parabola for opaque objects, and prismatic illuminator 

 and usual mirror for transparent objects. 



Another representation exhibited shows an arrangement in which 

 the body has no motion vertically. Its halves when separated are 

 capable of a partial rotation around the centre of their length, on two 

 axes carried by tbe racks that separate them on tm-ning the large mill 

 from " Mono " to " Bino." 



Two radius bars (now much lighter, having nothing to support) 

 affixed to the stage, alter the convergency of the bodies when the stage 

 is moved to or from the object-glass, and thus at once make the Horopter 

 coincide with the Focus of any object-glass used, and the whole is held 

 in position by a clamping screw. The stage has a further and finer 

 motion for focussing to distinctness. 



Note. — Mr. Holmes for-warded to us such a multiplicity of drawings that it 

 would have been as inconvenient as undesirable to rei)roduce them all. Never- 

 theless, we have allowed his reference to the figures in the ' British and Foreign 

 Mechanic ' to stand, as it helps to further explain his views. Our plate has been 

 constructed from his sketches and photographs, as a whole, and is, we think, suffi- 

 ciently explanatory of the principle adopted by Mr. Holmes. — Ed. M. M. J. 



