have yet penetrated the domain of domestic life; but when we consider electricity, as we hope to do in a future part, we intend to study this question fully, for there is no doubt that electricity is becoming more and more adapted to our daily life. [Illustration: Fig. 93.—An elementary Photometer.] The measurement of intensity of light is called _Photometry_, and the instruments used are _Photometers_. Bunsen’s instrument consists of a screen of writing-paper, saturated in places with spermaceti to make it transparent. A sperm candle is placed on one side, and the light to be compared on the other. The lights are provided with graduated bars, and these lights are then removed farther and farther from the screen till the spots of grease are invisible. The relative intensities are as the squares of the distance from the screen. [Illustration: Fig. 94.—The soap-bubble.] We append a table showing the comparative cost of light given by Dr. Frankland at the Royal Institution some few years ago. The standard of comparison was 20 sperm candles burning for 10 hours at the rate of 120 grains an hour:— _s._ _d._ Wax 7 2½ Sperm Oil 1 10 Paraffin 3 10 Spermaceti 6 8 Coal Gas 0 4½ Paraffin Oil 0 6 Tallow 2 8 Cannel Gas 0 3 Rock Oil 0 7⅔ There are many other interesting experiments connected with Light,—Spectrum Analysis, etc., etc.,—all of which we will defer for a time until we have examined the Eye and some effects produced upon it by Light, illustrated by numerous diagrams in the pages next following. CHAPTER X. VISION AND OPTICAL ILLUSIONS—THE EYE DESCRIBED—ACCOMMODATION OF THE EYE—CHROMATIC ABERRATION—SPINNING TOPS. The eye is an optical instrument that may be compared with those constructed by physicists themselves; the _media_ of which it is composed have surfaces like those which enter into the construction of optical instruments. It was Kepler who at the end of the eighteenth century discovered the passage of light into the eye. Soon after the discovery of the inner chamber he found that the eye realized the conditions that Porta had combined to obtain the reflection of external objects.