THE WOODWARD ILLUMINATOR. 167 



with the slide on which the object is mounted. The 

 details of the plan will be understood by the diagram 

 on next page, in which the glass prism is seen in section 

 just beneath the object slide F F. Just below it is an- 

 other right-angled prism, of the same dimensions, made 

 of brass ; the section of this prism is indicated by dark 

 shading in the diagram . The right-angles of both prisms 

 are truncated, and the facets are cemented together in 

 such a manner that the long sides of the prisms are par- 

 allel. The brass prism slips transversely in a groove in 

 the top of a holder, C, which is fitted into the sub-stage 

 of the microscope. D D is a blackened brass screen 

 held in position by two brass arms, one of which is 

 shown in the figure. This screen is parallel to the adja- 

 cent face of the glass prism, and has in it a small cir- 

 cular aperture, E, about the size of a large pin hole. 

 The side of the glass prism next the scree'n is covered 

 with black paper, in which is a corresponding pin hole. 

 The two pin holes are so placed that a beam of parallel 

 white sunlight (r) passing through both will be per- 

 pendicular to the sides of the glass prism on which it 

 impinges. 



" To use this apparatus it is adjusted in the sub-stage 

 ot the microscope, a drop of oil of cloves is placed on 

 the upper face of the prism, the glass slide F F, on 

 which the object is mounted in Canada balsam under the 

 usual thin cover, Gr, is placed on the stage, and the sub- 

 stage is racked up until the drop of oil of cloves is 

 spread out into a thin layer, I. 



" The object being thus arranged, it is evident that if 



