ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 281 



is a crosspiece T, which at one end presses directly against the frame 

 B, acting at the other by means of a weak spring. This is designed 

 to keep the tube firm in its bearings, to prevent any rotation, and to 

 make it impossible for the tube to leave the frame altogether. The 

 instrument is carried by the wooden handles H H. 



In using the instrument it is placed gently on the spherical surface to 

 be determined, which, if a thin lens, is supported on a ring of the same 

 diameter as the spherometer ring. The Microscope is focused, and 

 a reading is taken on the scale by the micrometer. The zero-point 

 having been found by placing the spherometer on a true plane surface, 

 and the difference of readings being = h, then R the radius of the 



sphere = — ^ — where r is the radius of the ring. 



A it 



A better method of determining h is to dispense with the plane 

 surface, and to take half the difference between the readings for the 

 spherical surface, and for another spherical surface of exactly equal and 

 opposite curvature. In this way it is possible to eliminate the error 

 due to the fact that a ring which has not a perfectly fine edge rests 

 upon a concave surface with a greater diameter, and upon a convex 

 surface with a less diameter than the measured diameter of the ring. 



If such an equal and opposite surface cannot be obtained, the 

 curvature of another lens of nearly the same radius which has an equal 

 and opposite surface is first determined in the above way ; this is com- 

 pared with the curvature as determined by the aid of a plane surface, 

 and so the error for a lens of nearly the given curvature is ascertained. 

 The given lens is then measured by means of a plane surface, and the 

 known correction applied in estimating the value of h. 



Galland-Mason's Microphotoscope. — If it is true that the world 

 sometimes knows nothing of its greatest men, it would appear to be also 

 true that the world sometimes may be ignorant of its greatest inventions. 

 At any rate, although we are always on the look-out for all that is 

 novel, and much that is curious in microscopical matters, we have only 

 now become acquainted with Mr. E. Galland-Mason's patent for the 

 " Microphotoscope." The instrument which the patentee gives this 

 name consists of a pair of spectacles with a number of microphotographs 

 arranged along the upper part of the rims, and placed in front of minute 

 magnifying glasses by which they are made visible to the wearer of the 

 spectacles. The rims being detachable, the microphotographs (of 

 written or printed matter, maps, or other objects) can be changed as 

 desired. As the patentee says, " a lecturer might have his lectures 

 photographed and placed in the rim of his spectacles, an actor his plays, 

 a lawyer his briefs, a clergyman his sermon, a tourist, maps, views, and 

 plans of the country through which he travelled, a shopkeeper a ready 

 reckoner, calendar, &c, a timber merchant cubes, measurements and 

 rules, and so forth." 



In the first patent, which was taken out in 1884,* the patentee had 

 provided a separate lens for each microphotograph. This he subsequently 

 found to be superfluous, and in the following year he obtained a second 

 patent f for the " Improved Microphotoscope " in which only one lens is 

 used. There are occasions, and this is one of them, when (like the 



* 1884, 8th January, No. *J12. t 1885, 24th January, No. 1027. 



