( 44 ? ) 
N. B Care muft be taken that the very end of the Red, 
and the Extremity of the Violet be taken in by the Lens; 
otherwife the Spetfriim will not be perfectly white at the 
Glafs’s Focus. 
There is no fix’d Difiance of the Prifm from the Lens, 
but it ought to be brought fo near the Prifm that the two 
Ends of the S'peBrutn may fall nearer the Axis of the Lens 
than the Edges of the Lens ; becaufe there the Refratfiion 
is notfo regular. 
Behind the Lens L, which made the Colours converge 
into White at the diftindt Bafe or Focus F, i plac’d the 
Lens /, which made the White be at f the diftincft Bafe 
of the two Glafies combin’d ; and the Experiment fucceed- 
ed as before. Fig. 17. 
When the Paper was held in the Focus of the Lens, fo as 
to receive the white Image of the colour’d Spectrum proje- 
cted by the Lens ; if with a Card I intercepted the red Ray, 
the White appear’d ting’d with Purple, and if 1 intercepted 
the Violet or purple Ray, or both,/ the White appear’d 
ting’d with Red ; and if the Red was intercepted at the 
fame time, the Spetfrum appear’d to be a Mixture of Yel- 
low, Green and Blue. If any fingle Colour was fuffer’d 
to fall upon the Lens, the reft being intercepted, that Co- 
lour wou’d continue the fame : only it wou’d be more 
intenfe in the Focus of the Lens. 
§ 
EXPERIMENT VII. 
I took a Board (Fig. 18.3 qh s which (load reclining on 
a Prop t, having an Hole of a Quarter of an Inch Diame- 
ter at h, and behind it a Prifm B fupported on two 
Props, as above-mention’d, foasto turn eafily* about its 
Axis; and having fet this Board on the Ground with the 
Prifm behind it at B ; by turning the Prifm A C about its 
Axis, l firft made the red Ray of the colour’d Speclrum 
Zzz i pafs 
