. The microscope; a simple handbook. Microscopes. 44 THE MICROSCOPE Belatiye intensitiea of different sources of Ulumination, the ordinary type is unsatisfactory unless used with a ground glass or tissue paper in front of it. A form of 1/2-watt bulb called the " Grid " is a good light, as the filaments, when looked at from the correct side, appear as a fairly large ribbon of almost homogeneous light. The " Pointohte " electric arc is esctremely good for the highest power work. The incandescent gas mantle lamp is a useful iUuminant, and a modification of this, heated by a me
. The microscope; a simple handbook. Microscopes. 44 THE MICROSCOPE Belatiye intensitiea of different sources of Ulumination, the ordinary type is unsatisfactory unless used with a ground glass or tissue paper in front of it. A form of 1/2-watt bulb called the " Grid " is a good light, as the filaments, when looked at from the correct side, appear as a fairly large ribbon of almost homogeneous light. The " Pointohte " electric arc is esctremely good for the highest power work. The incandescent gas mantle lamp is a useful iUuminant, and a modification of this, heated by a methylated spirit lamp, is an excellent light for those who have not gas or electricity. The relative intensities of a similar size small area of difierent iUuminants are approximately according to the following table taken from Mr. A. P. Trotter's book on Illtiminating Engineering : Candle . . 2i Daylight (blue sky) 2 FaraEEm lamp . 4 to 9 Incandescent gas mantle 50 Carbon electric filament 300 Metal electric filament 1,000 1/2-watt electric bulb . 5,250 " Pointolite " electric bulb . 12,000 Arc lamp . . â 80,000 to 110,000 Direct light from the sun . , 800,000 From the above table it is evident why ordinary daylight is not sufficiently powerful for high-power microscope work. The sun, even in a clear climate, requires the use of a heUostat, and the arc lamp requires a special equipment. It is of great advantage to use a very in- tense light modified with the neutral tint J. w !B=^ I wedge moderator described on page 32. iPt?^^^ I The brightness of the light can then be perfectly regulated to meet requirements. This advantage of a very powerful iUu- minant has been referred to in coimection with substage condensers and dark-ground illuminators, but care should be exercised in its use. When direct light _ is being used through a con- denser, it is damaging to the eyes if too strong a light is em- ployed. A strong iUuminant is necessary for high-power dark- ground or op
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