ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



609 



conditions necessary for obtaining minimum deviation. Although several 

 mechanical arrangements have been designed for meeting the difficulty, 

 none of them appeared applicable to his requirements, as they either 

 require too much space or fail in precision. He believes that his method 

 is novel. It is known that, if the prisms are once in the desired position, 

 their edges being truly vertical, their bases will form the sides of a 

 regular polygon inscribed in a circle. If one of the half-prisms be fixed, 

 the author shows that the displacement of the 

 centre of the circle is along a certain straight 

 line, and that, corresponding to a displacement 

 of the centre, the bases of the prisms must 

 always be tangents to a circle of certain radius 

 described with that centre. The author shows 

 how, by means of pulleys and flexible cords, 

 this adjustment can be readily obtained. 



(6) Miscellaneous. 



Gotch Ophthalmic Spinthariscope. — This 

 instrument (fig. 71), which is made by the firm 

 of R. and J. Beck, has been designed by Pro- 

 fessor Gotch, of Oxford, its purpose being to 

 afford a ready means of testing the retinal 

 excitability of the dark adapted eye and the 

 alterations in retinal sensitiveness which are 

 produced by light. The instrument contains a 

 small quantity of radium held on the further 

 side of an adjustable opaque pointer, an adjust- 

 able fluorescent screen of zinc-blende, a series 

 of diaphragms for limiting the field of view, 

 and a lens for focusing the scintillating flashes 

 which occur on the screen through the ema- 

 nations from the radium. The main features 

 of the instrument are as follows. A handle 

 F allows the instrument to be held in the 

 hand of the observer without danger of altering 

 the adjustments. The lens A through which 

 the fluorescent screen E is viewed is capable 

 of being adjusted by revolving the milled 

 collar B so as to secure accurate focusing. The 

 pointer carrying the radium lies within the tube 

 in front of the screen E, and is fixed upon 

 a graduated slide C, by means of which it* can 



be withdrawn from the centre of the tube to any desired distance up 

 to 20 mm. Between the pointer and the focusing lens is a second 

 graduated slide D,which is provided with a series of circular apertures of 

 different sizes, so that the visual field can be varied in size. When 

 the slide D is pushed quite in, the fluorescent screen is completely 

 covered, so that when not in use it is protected from light entering 

 the tube through the eye-piece. The fluorescent screen E can be 



Fig. 71. 



