Optics and light

One of the greatest obstructions to the construction of large achromatic telescopes is the difficulty of procuring large discs of flint glass of a uniform density — of good colour, and free from veins.
— Thomas Dick, quoted by Henry King, The History of the Telescope, Dover edition, 1979, 176.

It has sometimes happened that in the development of certain branches of physical science practice has preceded theory … Thus the fundamental law of refraction, attributed to Snell, according to which the ratio of the sines of the angles of incidence and refraction is constant for any two media, is usually dated as the year 1621; but Galileo had constructed refracting telescopes, on a principle which may still sometimes be seen in opera glasses, some ten years before this date.
— A. J. Berry, From Classical to Modern Chemistry, 1954, Dover edition 1968, 82.

A 40-foot telescope should only be used for examining objects that other instruments will not reach. To look through one larger than required is a loss of time, which in a fine night, an astronomer has not to spare.
— Sir William Herschel, quoted by Henry King, The History of the Telescope, Dover edition, 1979, 133.

We can give such figures to transparent bodies, and dispose them in such order with respect to the eye and the objects, that the rays shall be refracted and bent to any place we please; so that we shall see an object near at hand or at a distance, under any angle we please. And thus from an incredible distance we may read the smallest letters and may number the smallest particles of dust and sand … the sun, moon and stars may descend hither in appearance …
— Roger Bacon, quoted by Henry King, The History of the Telescope, Dover edition, 1979, 27.

If an object is placed in a dense spherical medium of which the curved surface is turned towards the eye and is between the eye and the centre of the sphere, the object will appear magnified.
— Abu Ali Al-Hasan Ibn Al-Haytham (often called Alhazen) (965-1038), quoted by Henry King, The History of the Telescope, Dover edition, 1979, 25.

Socrates: Suppose you are arrested for a debt,
We’ll say five talents, how will you contrive
To cancel at a stroke both debt and writ?

Strepsiades: I’ve hit the nail
That does the deed, and so will you confess.

Socrates: Out with it!

Strepsiades: Good chance but you have noted
A pretty toy, a trinket in the shops,
Which being rightly held produceth fire
From things combustible —

Socrates: A burning glass, vulgarly called —

Strepsiades: You are right; ‘tis so.

Socrates: Proceed.

Strepsiades: Put the case now your whoreson bailiff comes,
Shows me his writ
— I, standing thus, d’ye mark me,
In the sun’s stream, measuring my distance, guide
My focus to a point upon his writ,
And off it goes in fumo! [in smoke]
— Aristophanes (c. 444 – c. 380 BCE), The Clouds, translated by T Mitchell, 1911.

… it is easy to see that the light energy per unit volume of space is exactly proportional to the number of photons in that volume.

Electrical theory, however, teaches us to regard energy as being spread continuously through space, not concentrated in the isolated points which happen to be occupied by photons. How are we to reconcile this with our supposition that the energy is merely the aggregate energy of individual photons occurring at isolated points?

The ordinary theory of gases points the way. It shows us a gas as a number of bullet-like projectiles its molecules. The mass of the gas is concentrated wholly in the few points of space which happen to be occupied by molecules. Nevertheless when we speak of the density of the gas, we suddenly change our picture; we form, so to speak, and out-of-focus picture in our minds in which the separate molecules are blurred into a continuous cloud, and what we describe as the density of the gas is merely the density of this blurred cloud, which we see spread continuously through space.

When we put this picture back into sharp focus, we see the separate molecules again. We see that the true density of matter varies abruptly from point to point it is large here, where there happens to be a molecule, and zero at an adjacent point where there is no molecule. Yet our former conception of a density which varied from point to point still retains a perfectly precise and clear-cut meaning. It is this: if we take a tiny fragment of space surrounding the point P, the chance of our finding a molecule inside it is exactly proportional to the density at P.

So, when we picture a beam of light as a shower of bullet-like photons, we must suppose that the density of the light-energy at each point of space gives a measure of the chance of our finding a photon there.
— Sir James Hopwood Jeans (1877 – 1946), The New Background of Science, Cambridge University Press, 1933, 221 – 2.

The idea of such a device can be traced back to Aristotle (384 – 322 BCE). In his Problemata he described a simple camera obscura that had a small pinhole. Other authors to mention the apparatus in detail include Leonardo da Vinci (1452 – 1519) in Codice Atlantico and Giovanni B. della Porta (Magiae Naturalis, 1553). Fifteen years later Daniel Barbaro showed that substituting a lens for the pin hole could give a brighter image.
— Con Tanre, Alan Davies and Peter Stanbury, The Mechanical Eye, Sydney: Macleay Museum, 1977, 11.

First, a piece of glass he coated
With collodion, and plunged it
In a bath of lunar caustic
Carefully dissolved in water —
There he left it certain minutes.

Secondly, my Hiawatha
Made with cunning hand a mixture
Of the acid pyro-gallic,
And of glacial-acetic,
And of alcohol and water —
This developed all the picture.
— Lewis Carroll, Hiawatha’s Photographing.

You will find an index to this blog at the foot of this link. Please be patient: I am pedalling as fast as I can.