"The litmus paper is still the litmus paper，" he enunciated in the formal manner of the lecturer. "I have not changed it into something else. Then what did I do？ I merely changed the arrangement of its molecules. Where， at first， it absorbed all colors from the light but red， its molecular structure was so changed that it absorbed red and all colors except blue. And so it goes， ADINFINITUM. Now， what I purpose to do is this." He paused for a space. "Ipurpose to seek--ay， and to find--the proper reagents， which， acting upon the living organism， will bring about molecular changes analogous to those you have just witnessed. But these reagents， which I shall find， and for that matter， upon which I already have my hands， will not turn the living body to blue or red or black， but they will turn it to transparency. All light will pass through it. It will be invisible. It will cast no shadow."A few weeks later I went hunting with Paul. He had been promising me for some time that I should have the pleasure of shooting over a wonderful dog--the most wonderful dog， in fact， that ever man shot over， so he averred， and continued to aver till my curiosity was aroused. But on the morning in question I was disappointed， for there was no dog in evidence.
"Don't see him about，" Paul remarked unconcernedly， and we set off across the fields.
I could not imagine， at the time， what was ailing me， but I had a feeling of some impending and deadly illness. My nerves were all awry， and， from the astounding tricks they played me， my senses seemed to have run riot. Strange sounds disturbed me. At times I heard the swish-swish of grass being shoved aside， and once the patter of feet across a patch of stony ground.
"Did you hear anything， Paul？" I asked once.
But he shook his head， and thrust his feet steadily forward.
While climbing a fence， I heard the low， eager whine of a dog， apparently from within a couple of feet of me； but on looking about me I saw nothing.
I dropped to the ground， limp and trembling.
"Paul，" I said， "we had better return to the house. I am afraid I am going to be sick.""Nonsense， old man，" he answered. "The sunshine has gone to your head like wine. You'll be all right. It's famous weather."But， passing along a narrow path through a clump of cottonwoods， some object brushed against my legs and I stumbled and nearly fell. I looked with sudden anxiety at Paul.
"What's the matter？" he asked. "Tripping over your own feet？"I kept my tongue between my teeth and plodded on， though sore perplexed and thoroughly satisfied that some acute and mysterious malady had attacked my nerves. So far my eyes had escaped； but， when we got to the open fields again， even my vision went back on me. Strange flashes of vari-colored， rainbow light began to appear and disappear on the path before me. Still， I managed to keep myself in hand， till the vari-colored lights persisted for a space of fully twenty seconds， dancing and flashing in continuous play. Then I sat down， weak and shaky.
"It's all up with me，" I gasped， covering my eyes with my hands. "It has attacked my eyes. Paul， take me home."But Paul laughed long and loud. "What did I tell you？--the most wonderful dog， eh？ Well， what do you think？"He turned partly from me and began to whistle. I heard the patter of feet， the panting of a heated animal， and the unmistakable yelp of a dog. Then Paul stooped down and apparently fondled the empty air.
"Here！ Give me your fist."
And he rubbed my hand over the cold nose and jowls of a dog. A dog it certainly was， with the shape and the smooth， short coat of a pointer.
Suffice to say， I speedily recovered my spirits and control. Paul put a collar about the animal's neck and tied his handkerchief to its tail. And then was vouchsafed us the remarkable sight of an empty collar and a waving handkerchief cavorting over the fields. It was something to see that collar and handkerchief pin a bevy of quail in a clump of locusts and remain rigid and immovable till we had flushed the birds.
Now and again the dog emitted the vari-colored light-flashes I have mentioned.
The one thing， Paul explained， which he had not anticipated and which he doubted could be overcome.
"They're a large family，" he said， "these sun dogs， wind dogs， rainbows， halos， and parhelia. They are produced by refraction of light from mineral and ice crystals， from mist， rain， spray， and no end of things； and I am afraid they are the penalty I must pay for transparency. I escaped Lloyd's shadow only to fetch up against the rainbow flash."A couple of days later， before the entrance to Paul's laboratory， Iencountered a terrible stench. So overpowering was it that it was easy to discover the source媋 mass of putrescent matter on the doorstep which in general outlines resembled a dog.
Paul was startled when he investigated my find. It was his invisible dog， or rather， what had been his invisible dog， for it was now plainly visible. It had been playing about but a few minutes before in all health and strength.
Closer examination revealed that the skull had been crushed by some heavy blow. While it was strange that the animal should have been killed， the inexplicable thing was that it should so quickly decay.
