ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 585 



places of decimals is readily obtained, and with a lens an approxima- 

 tion to the fourth place is possible. The prisms of the compensator 

 are revolved by a pinion acting on circular racks, the milled head of 

 the pinion being shown at t. A drum e moving with the lower 

 prism is graduated from to 60, and back to 0, the graduations 

 showing the angle z for every 3 degrees. 



The instrument is attached to a metal base, and has a concave 

 mirror g. In its normal position for observation it is inclined as 

 shown in fig, 115, but in order to insert the fluid between the prisms, 

 the telescope, sector, &c., can be turned completely away from the 

 observer so that the upper end c of the sector nearly touches the table, 

 the hypothenuse surface of the lower prism being then horizontal. 

 This is accomplished by the sector not being fixed to the support G, 

 but to an axis passing through it, the prisms with the alhidade being 

 on another axis within the former. Thus the prisms and index can 

 be moved together on the inner axis, or the whole sector, together 

 with the telescope, can be moved on the inner axis, carrying with 

 them the prisms and index. 



The following are the directions for use issued with the instrument 

 by Dr. Zeiss, of Jena, by whom it is made : — 



" On removing the instrument from its box (taking hold of it by 

 the foot and support G only) it should be placed so that the sector 

 with the telescope is turned away from the observer, the prisms C 

 being towards him. After taking out the small wood, or cork, wedge 

 (used for security in transit) the movable prism should be slipped 

 off by slightly pressing down the spring and drawing it backwards ; 

 the surfaces of the two prisms which come into contact are thus 

 free. 



After the prisms have been thoroughly cleaned, and the hypo- 

 thenuse surface of the fixed prism brought into a horizontal position 

 by turning the alhidade, a drop of the fluid to be examined is to be 

 placed in the centre of the prism by a glass rod, the movable prism 

 being replaced by pressing the spring down with the finger. 



The sector with the telescope is now to be turned up so that the 

 eye-piece is towards the observer, and the alhidade brought to the 

 beginning of the scale. 



Looking through the telescope, the mirror is adjusted so that the 

 whole field of view is uniformly illuminated, and the eye-piece 

 drawn out till the cross threads are seen sharply defined. 



The alhidade is then moved forwards till the lower half of the field 

 of view is obscured, and the screw turned until the boundary between 

 light and dark becomes a line as colourless as possible. By again 

 turning the alhidade this line is adjusted to lie along the two adjacent 

 points of intersection of the double cross threads. 



The position of the alhidade-index on the graduated arc, and the 

 position of the drum, are then to be read off on the respective scales, 

 a lens being preferably used for the former. After further turning 

 the screw till the boundary line a second time becomes colourless, it 

 is again adjusted on the cross threads, and the sector and drum 

 read off. 



