. The Ontario high school physics. ppears. This is a simple and very useful photometer. The block ofparaffin should be viewed through atube, using a single eye. For the block of paraffin one maysubstitute a wooden prism having twofaces covered with unglazed paper, andthe edge being turned towards theexperimenter. All photometric work should be done in a darkened room,and the eyes should be shielded from the direct light from thelamps which are being compared. There will usually be diffi-culty in adjusting the photometer due to a difference in thecolour of the lights. This cannot be avoided, ho
. The Ontario high school physics. ppears. This is a simple and very useful photometer. The block ofparaffin should be viewed through atube, using a single eye. For the block of paraffin one maysubstitute a wooden prism having twofaces covered with unglazed paper, andthe edge being turned towards theexperimenter. All photometric work should be done in a darkened room,and the eyes should be shielded from the direct light from thelamps which are being compared. There will usually be diffi-culty in adjusting the photometer due to a difference in thecolour of the lights. This cannot be avoided, however. 340. Verification of the Law of Inverse Squares. To do this let us use the Joly photometer (Fig. 329). Place 1 candle at one end of a board and 4candles at the other. Nowmove the photometer until theline between the paraffin blocksdisappears, and measure itsdistance from the 1 candle andthe 4 candles. The latter will I, Fig. 328. -Joly Diffusion Photo-meter, consistiiiK of two similarblocks of paraffin, set close Fio. 3-2!).—If the blocks are equally illumi-nated the 4 candles are twice asfarfiomthe photometer as the single candle. 298 PHOTOMETRY be twice the former. Next, replace the 4 candles by 9 andadjust as before. The distance from the 9 candles will be 3times that from 1. Thus if tlie distance is doubled the illumination is reducedto 5, since it requires 4 times as many candles to produceequality. In the same way if the distance w^ere n times asgreat we should require n^ candles to produce an illuminationequal to that given by the single candle. 341. Standards of Light. By the photometer we canaccurately compare the strengths of two sources of light, butto state definitely the illuminating power of any lamp weshould express it in terms of some fixed standard unit. Wehave definite standard units for measuring length, mass, time,heat, and most other quantities met with in physics; but noperfectly satisfactory standard of light has yet been devised. The
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