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, I encountered a terrible stench.  So overpowering was it that it was easy to discover the source—­a 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.

“The reagents I injected into its system were harmless,” Paul explained.  “Yet they were powerful, and it appears that when death comes they force practically instantaneous disintegration.  Remarkable!  Most remarkable!  Well, the only thing is not to die.  They do not harm so long as one lives.  But I do wonder who smashed in that dog’s head.”