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William Herschel | Philosophical Transactions of the Royal Society of London | (1800)
Key Takeaways
Plain English Takeaway
Different colors of sunlight can heat things up or light them up in different ways, and some colors are better at heating than others.
Study Aim
The paper aims to test whether the ability of sunlight to heat and light up objects is spread equally among all the colors that make up sunlight. The author questions the common belief that every color in sunlight contributes the same amount to heating and lighting. He wants to find out if some colors are better at heating, while others are better at lighting.
Simply put: The study wants to see if some colors of sunlight heat things more than others.
Study Design
The author conducts experiments using sunlight passed through a prism, which splits the light into its different colors (prismatic colors). He uses various colored glass filters to observe how much heat and light each color produces. He also notes his sensations of heat and brightness when looking at the sun through these filters, and measures the effects on objects placed in the colored light.
Simply put: The researcher shines sunlight through a prism and colored glass to see which colors make things hotter or brighter.
Findings
The research demonstrates that the power to heat and the power to illuminate are not equally distributed among the colors of sunlight. Some colors, like those beyond the red end of the spectrum, produce more heat but less light. Other colors, such as those in the middle of the spectrum, are better at lighting but not as good at heating. The author concludes that radiant heat (heat that travels as waves, like sunlight) can be bent (refracted) differently from visible light. He suggests that this discovery could help improve how telescopes are used to view the sun safely and clearly.
Simply put: The study finds that some colors of sunlight heat things more, while others are better for seeing, and this could help make better sun-viewing tools.
Abstract
It is sometimes of great use in natural philosophy, to doubt of things that are commonly taken for granted; especially as the means of resolving any doubt, when once it is entertained, are often within our reach. We may therefore say, that any experiment which leads us to investigate the truth of what was before admitted upon trust, may become of great utility to natural knowledge. Thus, for instance, when we see the effect of the condensation of the sun's rays in the focus of a burning lens, it seems to be natural to suppose, that every one of the united rays contributes its proportional share to the intensity of the heat which is produced; and we should probably think it highly absurd, if it were asserted that many of them had but little concern in the combustion, or vitrification, which follows, when an object is put into that focus. It will therefore not be amiss to mention what gave rise to a surmise, that the power of heating and illuminating objects, might not be equally distributed among the variously coloured rays. In a variety of experiments I have occasionally made, relating to the method of viewing the sun, with large telescopes, to the best advantage, I used various combinations of differently-coloured darkening glasses. What appeared remarkable was, that when I used some of them, I felt a sensation of heat, though I had but little light; while others gave me much light, with scarce any sensation of heat. Now, as in these different combinations the sun's image was also differently coloured, it occurred to me, that the prismatic rays might have the power of heating bodies very unequally distributed among them; and, as I judged it right in this respect to entertain a doubt, it appeared equally proper to admit the same with regard to light. If certain colours should be more apt to occasion heat, others might, on the contrary, be more fit for vision, by possessing a superior illuminating power. At all events, it would be proper to recur to experiments for a decision.
Referenced In
StarTalk Show Notes
a month ago
Created: May 12, 2026
Cool to engage with "OG" 1800 papers like this.