of the Cassegrainian Telescope ^ 235 
The mirror being placed to receive 'the rays of the sun, I 
held a card at such a distance within the focus, that the lumi- 
nous circle formed by reflection was equal in diameter to the 
width of the card ; and having endeavoured to fix in my mind 
the degree of illumination, I suddenly removed the card far 
beyond the focus, and then brought it towards the mirror till 
the diameter of the' luminous circle was the same as before. 
In this position the degree of illumination appeared so far in- 
ferior to that before observed, as to admit of no hesitation in 
the decision. 
Before I proceed to detail the experiments which follow, I 
must premise that of the eight persons who at different times 
were employed in making the observations, six were perfectly 
unacquainted with the subject of inquiry, and two of the six 
distinguished in the column of observers by the letters C, and 
W, were servants. It will be seen, that I did not trust myself 
to make any observations till the fact I sought had been suf- 
ficiently established by others to leave no fear on my mind 
’ that I might be influenced by any preconceived opinion. 
I drew a line across a card, which line I divided into tenths 
of an inch. The mirror was placed at the distance of fourteen 
feet from a candle, and the card moved in the reflected light 
within the focus till the diameter of the luminous circle was 
equal to the width of the card, in which position an observer 
was directed to remark carefully the intensity of the illumina- 
tion. The card was then suddenly carried far beyond the focus 
and brought slowly towards it till the observer pronounced 
the intensity of the light to be the same as before, at which 
instant I noted the diameter of the luminous circle by means 
of the divided line drawn on the card ; the square of this 
H h 2 
