—M. G. J. Minnaert, The Nature of Light and Colour in the Open Air, 1893, translated by H. M. Kremer-Priest

I
Everything described in this book is within
Your own powers of understanding and observation.

The Dark Inner Part, 
The Bright Outer Part,
The Variability of the Colour of the Blue Sky.
Try to imagine that you are looking at a painting 
And admire the evenness and delicacy of the transitions.

The Blue Sky:
What is the explanation of this curious phenomenon?
What can be the cause of this wonderful blue?

Compare the blue of the sky with the skies
Of Italy during your holiday.
The Colours of the Sun, Moon and Stars:

It is difficult to judge the colour of the sun,
Owing to its dazzling brightness. Personally,
However, I should say it is decidedly yellow.

Be very cautious during these observations!
Do not overstrain your eyes!

Study the illumination … 
Compare the light … 
Compare the illumination … 
Compare the luminosity … 
Compare the light inside and outside a wood. 
Intercept one of these images by a piece of paper.

II
Above all study your surroundings intently.
Move your opera glasses gently a little to the left,
Then to the right, and back again to the left …

The colour of lakes,
The colour of puddles along the road,
Strong wind rising, grey sky.

One more reason to keep our eyes open.
Which are the feeblest stars
Perceptible by you? 

Practice on cold evenings.
Try to fill this gap.
Notice How much better you succeed with a little practice!

Estimate the strength of the ash-grey light
On a scale from 1 to 10.

Watch the separate breakers along the shore.

Imagine a pool of water in a hollow of the dunes …
Be sure to carry out this experiment!
It is as convincing as it is surprising!

Examine systematically the colours of the shadows!

Draw up a scale for the phosphorescence of the sea!

Everything is meant to be seen by you and done by you!

Always try to find the explanation.

This causes a very peculiar sensation, 
Difficult