Clue 1
Have you identified all six individuals pictured?
Clue 2
Searching their last names should make it easy.
Solution
The first letter of each first names spells out DALLAS.
Clue 3
They are, in order: Daniel Solander, Alfred Nobel, Lise Meitner, Liev Kreyberg, Anders Celcius, Svante Arrhenius.
Clue 1
Probably need to find something to fill the blanks, first...
Clue 2
Luckily, you can get that information somewhere else on the card.
Solution
The lines will spell out the name ALLEN MARK DAVIS.
Clue 3
If you haven't solved the front of the card yet, you should do so now.
Clue 4
The scientists' names from the front of the card will slot into these lines.
Clue 5
The colored lines, in order of the colored numbers, will give you what you need.
Clue 1
Does anything look familiar about these numbers?
Clue 2
To be fair, this puzzle is a much higher degree of difficulty than some others...
Solution
All the numbers are degrees in Farenheight. When converted into Celcius, they yield single digit numbers from 1-26, forming a simple alphabet. The resulting plaintext reads: "Their chief of European operations is known as Leviathan."
Clue 3
Anders, from the front of the card, may be able to help.
Clue 4
You may need to adjust your temperature.
Clue 1
Have you solved any other of the stamps yet?
Clue 2
This will be solved in much the same way.
Solution
In Norwegian, the words for 'bread' and 'page' are 'brod' and 'side' which combine into "Broadside."
Clue 3
Much like the second card, you'll need to think outside of English...