What are the differences between lenses? I know some are made of plastic and some from glass..Thanks!

Glass of certain formulations to get the different wavelengths of light to focus to the same point along with coatings to do the same
Look at a cutaway image of a lens system to see the construction
Like 11 elements in 7 groups if you want the exact composition of the lenses quite secretive with their recipes for glass or plastic elements

100% acetone is best as long as the lenses are removed...as long as the lenses are glass, not plastic or acrylic.

Hello russel and welcome to FixYa. Yes you can take the plastic layer off if you wish. They are just there for the manufacturing and shipping process to protect them from damage. I hope you found this helpful. Happy Easter.

Well, here are some facts about glass lenses that might help you reach a better decision.Glass is the only lens material that can be made into 100% optically correct sunglasses. For true vision, there is no comparison between glass and other lens materials.
Glass does not flex or compress. Whenever lenses are placed into an eyewear frame, they are held in place by pressure of some kind. Plastic lenses flex and that creates distortion; but that cannot happen with glass lenses.
Glass does not expand or contract in heat and cold in a significant way. No matter what the temperature outside, 100% optically correct sunglasses will remain 100% optically correct.
Glass is the hardest material, and the most scratch-resistant. Glass is the most durable, and longest-lasting sunglass lens material. However, it is not scratch proof. Sand can scratch glass, but it requires serious effort or trauma to scratch glass lenses.
Technology has made it possible to create a very lightweight glass lens. Today, it can be difficult to tell the difference between glass and plastic, even when holding a pair with glass lenses in one hand and a pair with plastic in the other hand. (Of course, the difference is clear when looking through the lenses. But the weight of the lens is no longer a dead giveaway as to which is which.)

The danger of shattering was once one of the major disadvantages of glass lenses. However, all glass lenses pass the "Drop Ball Test" for impact resistance - the same test that is administered to polycarbonate and CR-39 lenses. When today's polarized lenses tend to break in a similar pattern as a car windshield. The broken pieces of glass mostly stick to the polarizing film that is inside the lens.
These Spotters had been stepped on by the customer. Notice that the broken lens has retained its original curvature. The broken lens pieces were still so tightly held by the frame that we had to smash the broken lens out of the frame with a hammer (it took more than a few smacks, too!). This isn't to say in any way that glass lenses are equivalent to safety glasses. However, polarized glass lenses are much, much less likely to cause injuries today than they ever have been in the past.

Polycarbonate lenses are the most-used alternative material for modern ophthalmic lenses. Consumer interest in thinner, more attractive spectacle lenses led to the development of special lens materials that would bend light differently than conventional plastic lenses. These materials are called "hi-index".
Among the high index materials developed for eyeglasses is one material that has a number of very unique properties and that also provides special benefits to eyeglass wearers. It carries the generic name "polycarbonate" but is sometimes called something different by other manufacturer trade names.
Great for sports protective eyewear
A "must" for kids
Comes with FREE Scratch and UV Protection
Impact Resistant - Polycarbonate Surpass AR lenses are 10 times more impact resistant than plastic lenses.
Lighter - Polycarbonate Surpass AR lenses are up to 35% lighter and thinner than standard plastic lenses for greater comfort.
UV Protection - Polycarbonate Surpass AR lenses shield your eyes from virtually 100% of the sun's harmful UV rays.
Scratch Resistant - PolyCarbonate Surpass AR lenses are hardcoated for excellent durability and scratch resistance.
Easy Care - PENTAX EasyClean™ treatment resists dust and smudges, making your Polycarbonate Surpass AR lenses easy to clean.
Polycarbonate was the first plastic high index lens and still has one of the highest index ratings (1.59). High index advantages are included with every polycarbonate lens so there is never any extra charge for high index when polycarbonate lenses are ordered. Originally used primarily for industrial safety glasses, now used for children, sports wearers, or anyone requesting greater impact resistance in their lenses.

Hello,
The lens is made of glass so be very careful. There are no replacement glass lenses available. We manufacture our own plastic replacement lenses from a template and a sheet of clear plastic. 
The easiest option would be to buy a complete lower shell and replace that.
Sorry I can’t be more help than that.


If you need any more help then please do not hesitate to contact us:-


[email protected] or TEL +44(0)1202 620955
http://www.biastechnology.com/
 

not much difference other than the glass is probably going to last longer and not be scratched as easily. and it is probably more precise than a plastic lens

Never disassemble an eyepiece unless you know exactly how to put it back together. Only the outer glass lens ever needs cleaning.

Without knowing exactly what link of eyepiece you have (they are all different designs) it's impossible to tell you which way the internal lenses go.

Your best bet may be to simply purchase another eyepiece.

http://www.agenaastro.com/Telescope-Eyepiece-s/3.htm

This site sells replacement rival lids. They have plastic and glass ones. I would suggest you measure you old one and order a new one in glass. The added weight of the glass should cure your problems. Thanks Kevin PS if this helped you pls leave feedback. Link is below just cut and past into your browser.
http://www.marbeck.com/crock_parts_rival.html