In this paper , historical optical ( UBVRI ) data and newly observed data from the Yunnan Observatory of China ( about100 years ) are presented for BL Lacertae .
Large variations of \Delta U = 5 ^ { m } .12 , \Delta B = 5 ^ { m } .31 , \Delta V = 4 ^ { m } .73 , \Delta R = 2 ^ { m } .59 , \Delta I = 2 ^ { m } .54 , and color indices of U - B = -0.11 \pm 0.20 , B - V = 1.0 \pm 0.11 , V - R = 0.73 \pm 0.19 , V - I = 1.42 \pm 0.25 , R - I = 0.82 \pm 0.11 , and B - I = 2.44 \pm 0.29 have been obtained from the literature ; The Jurkevich method is used to investigate the existence of periods in the B band light curve , and a long-term period of 14 years is found .
The 0.6 and 0.88 year periods reported by Webb et al .
( 1988 ) are confirmed .
In addition , a close relation between B-I and B is found , suggesting that the spectra flattens when the source brightens .