Tin-: I'RKsiijkxt's audrkss. 855 



a thin film, which can be mounted on an ordinary microscope 

 slide, is sufficient for all purposes ; this ingenious plan of his 

 obviates all necessity for bottles and troughs. 



Illuminants for the microscope remain much the same ; the 

 common paraffin lamp is still the best for visual work, and, 

 both on account of its cheapness and steadiness, it is now being 

 more and more used for photomicrography ; at the same time 

 it can hardly be said to be displacing oxy-hydrogen for that 

 purpose. Daylight, though very largely employed on the 

 Continent, is not so much used in this country as formerly. 



Many attempts have been made to employ the electric light 

 for microscopical work, but the image of the hot filament does 

 not seem suitable for the purpose. The arc, however, is with 

 advantage used in the projection microscope. 



Concerning accessory apparatus, there is not much to note. 

 Photomicrography is steadily gaining ground, both for micro- 

 metry and for the delineation of objects ; it is probably owing to 

 this that no new cameras, or micrometers have appeared. 



There are one or two exceedingly simple yet very practical 

 little devices, which, although invented some years ago, are to 

 be seen, neither at the opticians, nor on microscopists' tables. 

 They have only to be known to be thoroughly appreciated. 

 Taking them in the order of their publication, the first is 

 Burch's micrometer.* This consists of a cap fitting over the 

 eye-piece, and containing either a neutral tint, or a silvered 

 diagonal with a hole in it. At right angles to this cap is a 

 light wooden rod, 10 inches long, holding a paper scale. As 

 you look through the cap you see the object through 

 the eye-piece, and at the same time the image of 

 the scale reflected on it ; this is a useful, accurate, and yet 

 the simplest micrometer ever designed. Another form of this 

 instrument consists of the same cap- with a large mirror on an 

 arm, at right angles to the cap; this is now known as the Abbe 

 camera, but priority of invention and description belong to Mr. 

 Burch. The second device is Michael's polarizei',t which con- 

 sists merely of a piece of opalescent glass fitted over the miiTor. 

 As spar prisms have become expensive of late years, such a 

 cheap and efficient substitute will be heartily welcomed. The 



* This Journal, Vol. v., p. 4.v, 1S78. 

 . t 'i lii=i Journal, Vol. i., 8.8., i'. o23, 1883. 



