Aberration

Discussion

introduction

what is aberration

types/causes

chromatic aberration

Chromatic aberration comes in two types: axial and longitudinal.

To reduce chromatic aberration, a higher quality optical device would use a special combination lens called an achromatic lens or achromat for short. The simplest such system consists of two lenses made of two different glasses: a converging lens made of crown glass (the very common kind of glass used in windows) and a diverging lens made of flint glass (the slightly fancier kind of glass used in chandeliers and crystal decanters). An apochromatic lens corrects for both chromatic and spherical aberration.

The apparent three dimensional appearance of this album cover is an optical illusion caused by chromatic aberration in the eye.

history is his story

Newton was interested in the history of optical illusions. Is what we see there really there? To this end, he experimented on himself in a way that should never be repeated. When he was 24 years old, he inserted a bodkin (a blunt needle used to thread ribbon through lace) into the space between his eyeball and socket.

Entry 58 from Newton's lab notebook described the one of these experiments. Spelling, capitalization, and punctuation rules were not well established in the Seventeenth Century, so some of this may look a bit odd to contemporary readers. Pen, ink, and paper were all difficult to come by (Newton had his own recipe for ink), so abbreviations were common as well. The letter "y" was often substituted for "th" so that "the" is written ye , "that" is written yt, and "them" is written ym.

58 I tooke a bodkine gh & put it betwixt my eye & ye bone as neare to ye backside of my eye as I could: & pressing my eye with ye end of it (soe as to make ye curvature a,bcdef in my eye) there appeared severall white darke & coloured circles r,s,t, &c. Which circles were plainest when I continued to rub my eye with ye point of ye bodkine, but if I held my eye & ye bodkin still, though I continued to presse my eye with it yet ye circles would grow faint & often disappeare untill I removed ym by moving my eye or ye bodkin.

Pressing the side of the needle against his eyeball made colored circles appear in his field of vision at a point opposite that of the needle. These circles, which can be colored solid or take on animated geometric patterns, are an example of a visual phenomena known as a phosphene — the sensation of light when there is no light. To confirm that the visions he was seeing were not formed by light, Newton repeated the experiment (Entry 59) in a darkened room.

59 If ye experiment were done in a light roome so yt though my eyes were shut some light would get through their lidds There appeared a greate broade blewish darke circle outmost (as ts), & wthin that another light spot srs whose colour was much like yt in ye rest of ye eye as at k. Within wch spot appeared still another blew spot r espetially if I pressed my eye hard & wth a small pointed bodkin. & outmost at vt appeared a verge of light.

The reason Newton did these experiments on himself wasn't because he was some thick headed frat boy. Rather, he was fascinated by the difference between objective reality and illusion (or even delusion). One of the ways we are often fooled is in the perception of color. He showed through a series of now famous experiments using glass prisms that white light, which up to that point was thought to be the purest form of light, was actually a blended form of light with different colors. Entry 7 from Newton's notebook…

7 Taking a Prisme, (whose angle fbd was about 60gr) into a Darke roome into wch ye sun shone only at one little round hole k, and laying it close to ye hole k in such manner yt ye rays, being equally refracted at (n & h) their going in & out of it, cast colours rstv on ye opposite wall. The colours should have beene in a round circle were all ye rays alike refracted, but their forme was oblong terminated at theire sides r & s wth straight lines; theire breadth rs being 2⅓ inches, theire length to about 7 or eight inches, & ye centers of ye red & blew, (q & p) being distant about 2¾ or 3 inches. The distance of ye wall trsv from ye Prisme being 260 inches.

What Newton saw projected on the wall of his darkened laboratory looked something like this.

A continuous spectrum running horizontally across the page

Near the end of Entry 6 in his notebook, Newton called it a "phantom".

And looking on it through the Prisme, it appeared broken in two twixt the colours, the blew parte being nearer the Prisme than the red parte. Soe that blew rays suffer a greater refraction than red ones. I call those blew or red rays &c, which make the Phantome of such colours.

Scary. Six years later, when he made the results of the prism experiment public, Newton had begun the transition from the Greek loanword "phantom" (and variations like "phantasm") to the Latin loanword "spectrum" — and used both interchangeably for awhile.

Comparing the length of this coloured Spectrum with its breadth, I found it about five times greater; a disproportion so extravagant, that it excited me to a more then ordinary curiosity of examining, from whence it might proceed….

But, to determine more absolutely, what Light is, after what manner refracted, and by what modes or actions it produceth in our minds the Phantasms of Colours, is not so easie. And I shall not mingle conjectures with certainties.

Why the switch? Both words had similar meanings in the Seventeenth Century — something ghostly or not of this world. Much like spelling and punctuation, scientific terminology wasn't systematized in the Seventeenth Century. It may well have been seen as a mark of proficiency to mix up spellings, punctuation placements, and word choices. (This was the time when the thesaurus was invented.) In the Twentifirst Century, however, scientific terminology is reasonably well organized and consistent and, for unrelated reasons, the word spectrum has lost all its supernatural connotations.

Because Newton was a bit of a mystic and seven is a number with mystical connotations, he divided the spectrum up into seven named colors. He identified these as the "primary colors" but later experiments have shown this notion to be wrong. The preferred term now is spectral colors or prismatic colors for the things Newton was naming. (The primary colors of red, green, and blue are discussed elsewhere in this book.) There are also many more than seven distinguishable colors of light in the visible spectrum — a point Newton makes clear near the end of this quotation.

red orange yellow green blue indico violet-purple

There are therefore two sorts of colours. The one original and simple, the other compounded of these. The Original or primary colours are, Red, Yellow, Green, Blew, and a Violet-purple, together with Orange, Indico, and an indefinite variety of Intermediate gradations.

 

spherical aberration

spherical

geometric aberrations

coma

distortion

astigmatism

field curvature