( 44 1 ) 
If the Plane AC be dill more oblique to H F, all the 
Light will be refle&ed, and there will be no colour’d 
Image or Speftrum made by Refraction at all. Fig. iz. 
But if it be held fo as to be more nearly perpendicular 
to the Sun Beam than in Fig 10. the whole Beam will in 
deed enter the Prifm ; but meeting with B C the lower 
Surface of the Prifm, or rather the Surface of the Air con- 
tiguous to it, fome of the Light will by the Plane B C be 
reflected to d <?, palling almoft perpendicularly thro’ A B; 
and the reft will emerge thro’ B C , and by Refraction 
make the imperfect Spectrum D E. See Fig. 10. 
If the Sun-Beam enter A C perpendicularly and in the 
middle of it, the Light will be all reflected as in Fig. i 3 . 
fome of it by the Plane B C to /?, and the reft by the 
Plane AB to p. But if the Beam fall nearer to A (ftill 
perpend icularly.) it will all be reflected by the Plane A B? 
if nearer to B, it will be all reflected by the Plane B C. 
In order therefore to have the colour d Spcffrum as it 
ought to be, care muft be taken that the emerging colou- 
red Light may make the fame Angle with the Plane B C , 
as the immerging Light does with the Plane AC; that is, 
the Angle AEH muft be equal to B DG, as was Paid be- 
fore, Fig. 9. which may alfo be feen on the enlightned 
Dull in the Air, But the beft way is to turn the Prifm on 
its Axis, and at the fame time look at the colour’d Spe- 
ttrum 3 which will rife and falland become longer or fhort- 
er as you turn the Prifm ; and between the Afcent and 
Defcent of the Image, it will appear ftationary : there 
ftop the Prifm, and the Reflection will be fuch as is requi- 
red for all the Experiments hereafter mention'd. 
In order to have the Prifm move freely on its Axis, 
and ftop any where, I fix’d each End of it into a trian- 
gular Collar of Tin, from the End of which came a Wire, 
which was the Axis of the Prifm produc’d ; and ft) I laid it 
on two wooden Pillars, with a Notch on theTop to receive 
Z z z the 
