I have what I believe is a Robbins Patent vernier tang sight. 

In Roy Marcot's book,  Remington Rolling Block Rifles, Carbines, & Shotguns SPORTING & TARGET RIFLES, on page 174 there is a Remington No 1 Creedmoor Long Range Rifle pictured with a Robbins Patent Vernier Tang Sight.

On pages 190-191 there is a Remington No 1 Short Range Rifle pictured with a Robbins Patent Mid-Range Vernier Tang Sight.

I am looking for any information regarding these Robbins Vernier sights. 

These sights are adjustable for windage.

The only marking on the sight is: Pat'd May 15, 1877. The name, Robbins, appears nowhere on the sight.

Were they used on rifles other than Remington?

Were they offered by Remington, or are they an aftermarket add-on? The base on the sights look like they are made for a Remington rifle.

Any information on these Robbins sights, such as where I might find a copy of the original Robbins Patent would be much appreciated.

Thank you for your consideration.

Regards,
Powderman

Thank you, John Boy. You nailed it. This is just the information I was looking for.I am tempted to remember you in my will. Again, thanks.

Regards,
Powderman

How about posting a pic of what they look like?

I also spent a lot of time trying to find the patent with Google, when by fluke I found this:

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No mention of Madison on the patent papers, but I did notice an advertisement in the publication which was published by E. Remington & Sons in 1880,mentioned by Tom Boy above, placed by a E.H. Madison, Practical Gunsmith for his services.

Perhaps this the Madison connected with the Robbins-Madison Vernier Sight mentioned in the same publication in the section on target shooting.

Might be that Robbins designed the sight and Madison produced/adapted it for Remington rifles of the time.