Lewis opens his book Tables Requisite to the table 'Mean Refraction' and using the altitude derived from Step C he finds that he has to subtract 24". Refraction is the bending of a light ray as it passes through the atmosphere. This bending results from the increase in the atmosphere's density as the light ray travels downward toward the observer. Refraction makes an object appear higher in the sky than it actually is. The light ray always appears to be bent upwards except when it is vertical, therefore, Lewis subtracted 24" from the apparent altitude found in Step C.
