ACCESSORY INSTRUMENTS. 



141 



At other times, when chara is being observed, the diagonal 

 plate may be made to press it close to the front by means of thin 

 strips of glass, or a wedge of cork, or even a folded 

 spring of thin whalebone. When either of these 

 instruments is used, it may perhaps be necessary 

 to remind the reader that the microscope must 

 be so far inclined as to be nearly horizontal. 

 Messrs. Smith and Beck adapt to the object- 

 plate of their large microscopes a strong steel 

 pin, upon which a spring-holder is made to fit; 

 this serves to keep the trough firm and to prevent 

 its falling off, even when the microscope is per- 

 fectly horizontal. This form of trough proved 

 very serviceable to Mr. Lister, in 1834, during 

 Fii'. 94. ^is investigations into the structure of some of 

 the higher orders of polyps, and will be found of 

 very great value to those who devote their attention to this 

 most interesting branch of scientific inquiry. 



Frog-Plate. — This consists of a plate of brass, a a, about six 

 inches in length, and two-and-a-half in breadth, and either of 

 the shape represented by fig. 95, or of the same breadth 



Fig. 95. 



throughout ; the former plan, first suggested by Mr. Goadby, 

 is adopted by Mr. Ross, the latter by Mr. Powell. At one end 

 it is provided with a plate of glass to cover either a square or 

 round aperture, b, made in the brass, which serves for laying 

 the frog's foot on. Around this aperture are placed four or 

 more studs, c c, for the purpose of securing the threads by 

 which the web of the foot is kept open; in Mr. Powell's 

 plate, a series of small holes answer the same purpose. Mr. 



