摩訶不思議な目の仕組み – ものが見えるとはどういうことか

目とは「光」という刺激を受け取る感覚器です。ここでは目の仕組みについてわかりやすく解説したいと思います。

「ものが見える」というのは目に光が入っているということ

普段、わたしたちは「ものが見える」ということを特別意識せずに生きています。しかし、ものが見えることには絶対に必要な要素が三つあります。

目・光・物体

ものが見えるとき、そこには必ず光があります。目は受容器、もしくは感覚器と呼ばれる動物特有の器官です。受容器とは動物が外からの刺激を受け取る器官のことです。目という受容器に光が入ることで、わたしたちは「ものが見える」と思うのです。

植物には目はありませんから、ものが見えることはありません。ただし植物も光を感じることはできます。植物と話すことはできませんので、植物がどのように光を感じているかはわかりませんが、それは動物が持つ視覚のような感覚ではないと想像されます。

光がなければものは見えません。完全な暗闇というのは滅多に存在しないと言ってよいでしょう。夜に電気を消していても、月明かりがありますし、新月の夜でも星明かりがあります。しかし、星明かり程度では、ものの輪郭がうっすらと見えるくらいでしょう。

そして、「ものが見える」には物体が必要です。物体そのものが光を発していて、その光が目に入る場合と、光を発していない物体が反射した物体が目に入る場合の2パターンがあります。前者は光源といって、太陽や電球やスマートフォンの画面がこれにあたります。後者は月や机や本やほとんどのものがそうですね。

物体は光を吸収することができるものが多いのですが、それぞれ吸収することのできる光の色が異なります。緑と黒と白の3色に塗られた傘があったとしましょう。太陽からはすべての色の光が届きます。そして、傘の緑の部分は緑色の光だけを反射していて、その緑色の光がわたしたちの目に入ることで「緑色だ」と感じることができるのです。赤いランドセルは赤色の光以外は吸収して、赤色の光を反射しているということなのです。

同じ傘の黒と白の部分はまたちょっと違います。黒はすべての色の光を吸収している部分なのです。なので、光がない夜の空は黒いのです。光があるときは、空気が青い光だけを反射させて、青い光が空全体に広がっているので、昼の空は青く見えます。

また白の部分は、すべての光を反射している部分です。すべての光が反射して合わさると白になるとはちょっとイメージがつきにくいですが、滝や波の水しぶきが白く見えるのは、水が白いからではありませんね。水は透明です。しかし、小さな空気の泡が多くあるとき、光が当たったら泡に反射して白く見えるのです。

なので、目と光と物体、この三つがそろってはじめて「ものが見える」ということになるのです。

わたしたちは過去を見ている

いまご説明したように、ものが見えるということは反射された光を見ているということなのですが、光がもっとも速度が速い物質なので、わたしたちは普段気にも止めていませんが、それでもある地点から別の地点へ行くのには時間がかかります。なので、見ている物体から反射した光がわたしたちの目に入る間にも、一瞬ですが時間がかかっています。なので、わたしたちが物質をみているとき、それはその物体の「過去を見ている」ということになるのです。

太陽と地球は1億4960万kmはなれています。この距離を光の速度で割ると、約8分になります。つまり、太陽から放たれた光が地球に届くまでには約8分間かかるということです。つまり、わたしたちが今見ている太陽は、8分前の太陽の姿なのです。なので、もしいまこの瞬間、太陽で大爆発が起こったとしても、わたしたちがそれを知るのは8分後になってしまうのです。

地球は天の川銀河という銀河の片隅にあります。天の川銀河の隣にある銀河はアンドロメダ銀河というもので、この銀河は地球から目視することもできます。もしアンドロメダ銀河に知的生命体がいて地球を観察できるような超強力な望遠鏡を持っていたとしたら、その知的生命体が見ている地球には恐竜が大地を闊歩していることでしょう。地球からアンドロメダ銀河に光が到達する時間はそれほどの時間がかかるのです。

光を受け取る目の仕組み

ものが見えるということがどういうことかわかったところで、光の受容器である目の仕組みについて考えていきましょう。

目には視細胞というものがあります。視細胞は光を感じることのできる細胞です。ミミズには目はありませんが、身体の表面に視細胞が散在しています。なので、全身で光を受容しているといえますね。しかし、ほとんどの動物は目という視細胞が1箇所に集まった光受容器官を持っています。

網膜には大きく分けて視細胞・連絡細胞・視神経という三つの種類の細胞があります。

まず、光を吸収すると視細胞が酵素を活性化させます。するとある物質の濃度が上がり、細胞膜の電位変化が起こります。その電気シグナルが連絡細胞を経由して、視神経に伝わり、さらにいくつかの神経を乗り継いで、最終的に大脳の後頭葉の視覚中枢に伝えられます。

光の刺激を受け取ることができるのは視細胞だけで、連絡細胞や視神経は刺激を受け取るはたらきはありません。

参考文献

・ リチャード・ドーキンス『ドーキンス博士が教える「世界の秘密」
・ 吉田邦久『好きになる生物学

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