A Catechism of Familiar Things; Their History, and the Events Which Led to Their Discovery. With a Short Explanation of Some of the Principal Natural Phenomena. For the Use of Schools and Families. Enlarged and Revised Edition.

CHAPTER XX.

LIGHT, LIME LIGHT, MAG­NE­SIUM LIGHT, ELEC­TRIC LIGHT, RAIN­BOW, PRISM, SPEC­TRUM, COL­ORS, PHO­TOG­RA­PHY, CAM­ERA OB­SCU­RA, STERE­OSCOPE, KALEI­DO­SCOPE.

Do you know some­thing about the na­ture of Light?

Light is a mere form of vi­bra­tion like sound, and like sound it re­quires some source to set this vi­bra­tion go­ing, and some medi­um to car­ry this vi­bra­tion as air car­ries sound.

Is not the air this medi­um?

No, it is sup­posed that there is an elas­tic flu­id called “ether” which per­vades all space and mat­ter, and if the molecules of a body are in mo­tion they have the pow­er of set­ting this ether in mo­tion. The move­ment thus pro­duced will ap­pear ei­ther as heat or light ac­cord­ing to its ve­loc­ity.

What sources of light do you know?

We are told that the prin­ci­pal source of light on earth is the sun, ei­ther di­rect­ly with its own beams or in­di­rect­ly by sup­ply­ing us with com­bustibles to pro­duce light; for oil, gas, can­dles, and most of the sub­stances used for pro­duc­ing light and heat when burn­ing are but send­ing forth in an­oth­er form the rays of the sun which were stored up in na­ture's econ­omy.

An­oth­er source of light is the re­sult of chem­ical ac­tion, such as the lime, mag­ne­sium, and elec­tric light. A third source of light is phos­pho­res­cence, as we see it in the glow-​worm and fire­flies.

What is the Drum­mond or Lime Light?

It is one of the most bril­liant of ar­ti­fi­cial lights. When a stream of oxy­gen and one of hy­dro­gen un­der pres­sure are brought to­geth­er and mixed with­in a few inch­es of the end of a blow­pipe, the mix­ture on light­ing burns with a col­or­less flame pos­sess­ing in­tense heat. If this flame be made to play up­on a ball of car­bon­ate of lime, the lime on be­com­ing white hot gives off a pow­er­ful in­can­des­cence.

_In­can­des­cence_, the glow­ing white­ness of a body caused by in­tense heat.

What is a Blow­pipe?

A tube, usu­al­ly bent near the end, ter­mi­nat­ed with a fine­ly-​point­ed noz­zle, for blow­ing through the flame of a lamp or gas-​jet, pro­duc­ing there­by a small con­ical flame pos­sess­ing in­tense heat. It is used in sol­der­ing sil­ver, brass, etc. A mix­ture of oxy­gen and hy­dro­gen when ig­nit­ed con­sti­tutes the hy­dro­gen blow­pipe, in­vent­ed by Dr. Hare of Philadel­phia.

What is Mag­ne­sium Light?

When the met­al mag­ne­sium is rolled out in­to a fine rib­bon and heat­ed to red heat it burns with a daz­zling light.

Which is the most pow­er­ful ar­ti­fi­cial light?

The so-​called Elec­tric light. This light, whether pro­duced by a se­ries of gal­van­ic cells or by dy­nam­ic pow­er, is the most bril­liant and use­ful.

What is a Rain­bow?

The rain­bow is that beau­ti­ful se­mi-​cir­cu­lar band or arc of dif­fer­ent col­ors in the clouds dur­ing the oc­cur­rence of rain in sun­shine. When the clouds op­po­site the sun are very dark and rain is falling from them, the rays of the sun are di­vid­ed by the rain­drops as they would be by a prism. There are of­ten two rain­bows at the same time, be­cause the pri­ma­ry bow is again re­flect­ed to an­oth­er lay­er of clouds.

What is a Prism?

A tri­an­gu­lar sol­id piece of glass, on which if a ray of light be cast it will be dis­tinct­ly di­vid­ed in­to the sev­en col­ors we see in a rain­bow. By this fact we see that white light is com­posed of dif­fer­ent rays which have dif­fer­ent re­flec­tive sus­cep­ti­bil­ities.

What is a Spec­trum?

It is this beau­ti­ful band of sev­en col­ors ob­tained by the re­frac­tion of a ray of light through the prism.

Whence come the col­ors in the ob­jects we see in na­ture?

They all come from light; ev­ery ob­ject has a pow­er to ab­sorb cer­tain rays and to re­flect oth­ers. A red cloth, for ex­am­ple, ab­sorbs all the oth­er col­ored rays ex­cept red, and this it gives off, thus ap­pear­ing red.

Why are the leaves of plants green?

Be­cause a pe­cu­liar chem­ical sub­stance called chloro­phyl, formed with­in their cells, ab­sorbs all oth­er rays of light, re­flect­ing on­ly blue and yel­low--which mix­ture pro­duces the dif­fer­ent green tints.

What is Pho­tog­ra­phy?

The word means “light draw­ing.” It is a mode of fix­ing on cer­tain sub­stances the lights and shades of any ob­ject by means of a lens in­sert­ed in a cam­era ob­scu­ra. This pro­cess was first called Da­guerreo­type from the name of the in­ven­tor, Da­guerre. A plate of cop­per thin­ly coat­ed with sil­ver is ex­posed to the va­por of io­dine, then placed in a cam­era ob­scu­ra, where an im­age of the ob­ject to be pre­sent­ed through a lens is cast up­on it. Am­brotype is the same ap­pli­ca­tion to glass. There are now dif­fer­ent vari­ations of method in the use of the same agents. Now pho­tog­ra­phy con­sists in tak­ing the im­ages on what is called a neg­ative--that is, a glass coat­ed with a sil­vered col­lo­di­on (gun-​cot­ton dis­solved in al­co­hol and ether) film. From this plate an­oth­er im­age is tak­en on sil­vered pa­per, which we call the pos­itive im­age. There are al­so oth­er chem­icals used in­stead of sil­ver.

What is a Cam­era Ob­scu­ra?

A small box or dark room in­to which the light is ad­mit­ted through a lens.

What is a Stere­oscope?

It is an in­stru­ment ex­hibit­ing the ef­fects and ad­van­tages of see­ing with two eyes. The in­stru­ment is so con­struct­ed that from a flat pic­ture we may see the sol­id body in its re­al­ity in na­ture.

What is a Kalei­do­scope?

An in­stru­ment in­vent­ed by Sir David Brew­ster, con­sist­ing of a tube with slips of re­flect­ing glass so ar­ranged in the in­te­ri­or that small beads, bits of col­ored glass, and sim­ilar things are, by re­volv­ing the tube, thrown in­to an end­less va­ri­ety of beau­ti­ful shapes.