=91. Light records.= The actual photographic records obtained by photometer and selagraph can at most be kept but a few months, unless they are toned or fixed. “Toning” modifies the color of the exposure materially, and changes its intensity so that it can not be compared with readings not fixed. It would involve a great deal of inconvenience to make all comparisons by means of toned strips and standard, even if it were not for the fact that it is practically impossible to obtain exactly the same shade in lots toned at different times. The field record, if carefully and neatly made, may well take the place of a permanent one. The form is the following: ═══════╤══════════╤═════════╤═══════╤════════╤════════╤══════════╤═══════ Day │ Hour │Formation│Station│Altitude│Exposure│ Group │Height │ │ │ │ │ │ │ │ │ │ │ │ │ │ ───────┼──────────┼─────────┼───────┼────────┼────────┼──────────┼─────── 14/9/04│ 12:00 M. │ Spruce │ Milky │2600 m. │N.E. 20°│Opulaster │1 foot │ │ │ Way │ │ │ │ „ │12:05 P.M.│ Spruce │ Moss │2500 m. │ Level │Streptopus│ „ │ │ │ Glen │ │ │ │ „ │12:15 P.M.│ Brook │Grotto │2500 m. │ E. 3° │ Filix │Surface │ │ b’nk │ │ │ │ │ ───────┴──────────┴─────────┴───────┴────────┴────────┴──────────┴─────── ═══════╤════╤════════╤════════╤═════╤═════╤═════════╤════════╤════════ Day │No. │ Length │Standard│Light│Base │Reflected│Transm’d│Absorbed │ │ of │ │value│value│ light │ light │ light │ │exposure│ │ │ │ │ │ ───────┼────┼────────┼────────┼─────┼─────┼─────────┼────────┼──────── 14/9/04│2:10│ 160 s. │ 3 s │.019 │ │ │ │ │ │ │ │ │ │ │ │ „ │2:12│ 240 s. │ 3 s. │.012 │ │ │ │ │ │ │ │ │ │ │ │ „ │2:13│ 360 s. │ 3 s. │.008 │ │ │ │ │ │ │ │ │ │ │ │ ───────┴────┴────────┴────────┴─────┴─────┴─────────┴────────┴────────