The Number 42 gained in popularity due to Douglas Adams' Hitchhikers Guide to the Galaxy, though the importance of the number existed long before and well after this book revealed the mystery of 42 to it's readers.
When light is refracted at 40-42 degrees to the viewer it forms a rainbow. A rainbow consists of 7 colors of visible light separated from white light. The 7 colors are related to the 7 luminaries, 7 chakras, 7 notes and so on. The rainbow appears in the bible after the flood of Noah as a sign from God that he would not destroy the world again. The rainbow is an important theme in The Wizard of Oz, with "Somewhere Over the Rainbow" as a signature song from the movie. The Land of Oz has been shown to be similar to both the Underworld and the Garden of Eden which makes the rainbow and number 42 related as well. An example of this relationship is found in the Egyptian belief in the 42 Principles of Maat, which is a list of 42 negative-confessions which the recently deceased must state during their trial in the Underworld.
The alchemical symbol for Tin appears to include the numbers 4 and 2.
Pythagorean numbers six and seven represent Light and Life, respectively. The six by seven matrix has fourty-two boxes. These correspond to Yahweh's fourty-two names, one for each combination of Light and Life.
The number 42
4+2 = 6, 4*2 = 8
Adding every number from 1 to 42 equals 903. (9+0+3 = 12 = 3)