216 
MR. W. H. BARLOW ON DIFFERENT MODES 
But we can hardly expect the lens to be so perfect an instrument as the reflector, 
not only from the obstruction of light caused by the zones, but from its being com- 
posed of separate pieces of glass, each of which has its own focus, which foci will 
coincide more or less according to the accuracy with which the instrument is made ; 
and it is doubtless from the want of mathematical exactness in the adjustment and 
curvature of the pieces of which it is composed, that a small surface of light, such as 
the lime ball, is observed to produce but little effect when placed in its focus, and 
that the observed divergence of the lens is greater than the computed. 
Taking the diameter of the lamp at 3 inches, the maximum horizontal divergence 
would be 5°, whereas in a revolution of 8 minutes the observed duration of the light 
was 7 seconds, making the observed angle of divergence 5° 15'. 
In the reflector the duration was 25 seconds, which gives the observed divergence 
18° 45', while the computed divergence is 19° 10', the computed in this instrument 
exceeding the observed. 
Yet as eight lenses may be applied to one light, as well as some additional appa- 
ratus which increases their illuminating power, it possesses an advantage in point 
of economy when applied to a lighthouse ; for with a lamp, consuming the oil of 
fifteen or sixteen Argand burners, we are enabled to illuminate eight times 5° 15' 
or 42° of the horizon, the full power of the light being (with the additional appa- 
ratus) equal to 1Q'4 reflectors 21 inches diameter; while a reflector frame, such as 
that at Beechy Head, consumes the oil of thirty burners to illuminate three times 
3 8° 45' or 56° 15' of the horizon, the full power of the light being equal to ten re- 
flectors ; whence it appears that the lenses illuminate three fourths of the number of 
degrees in the horizon which the reflectors do with the consumption of one half of 
the oil. 
But although the sum of the angles of maximum divergence of the eight sides of a 
system of lenses is equal to three fourths of the sum of the angles of maximum di- 
vergence of the three sides of a reflector frame in the horizontal direction, yet the 
vertical divergence of the reflectors is far greater than that of the lenses. 
The following are the maximum horizontal and vertical divergences of the reflector 
and lens. 
Max. Hor. Div. Max. Vert. Div. 
Reflector 18° 45' 31° 54' 
French lens .... 5 15 30 
The computed vertical divergence is less than this, but the observed horizontal 
divergence being found more than the computed, the 3° above stated would probably 
he found sufficiently correct in practice. 
Hence it appears that the advantage gained by the use of lenses over that of re- 
flectors, is not dependent upon their greater perfection as optical instruments, but 
from their using the light more economically, by reason of their having less divergence 
both horizontally and vertically, and illuminating much less space in the horizon. 
