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Achromatic design I think

http://en.wikipedia.org/wiki/Achromatic_lens

The adhesive you removed was Canada Balsam, should be availabe from sowm one who sells microscope slides as it is also used to fixing slide covers onto a slide.

The reason Canada Balsam is used, as opposed to mineral oil, is that he has roughly the same refractive index as crown glass (, so when you are making an achromatic doublet (crown glass {n=1.523} to extra dense flint glass {n=1.8ish}) or any lens system where you need to join lens surfaces then you can ignore the effect of the glue.

If you decide to refix with Canada Balsam then clean the lens surfaces very, very carefully and gently place the two lenses into contact making sure that ther are no, repeat no , air bubbles present. If you maje a pigs ear of it then you can dissolve the glue by using acetone (as you discovered). For bonding lens surfaces now, say inside a diving mask to give a spectacle correction, a UV setting glue is now used (if you cock that up it's a new pair of lenses).

ramseyoptom wrote:The adhesive you removed was Canada Balsam, should be availabe from sowm one who sells microscope slides as it is also used to fixing slide covers onto a slide.

The reason Canada Balsam is used, as opposed to mineral oil, is that he has roughly the same refractive index as crown glass (, so when you are making an achromatic doublet (crown glass {n=1.523} to extra dense flint glass {n=1.8ish}) or any lens system where you need to join lens surfaces then you can ignore the effect of the glue.

If you decide to refix with Canada Balsam then clean the lens surfaces very, very carefully and gently place the two lenses into contact making sure that ther are no, repeat no , air bubbles present. If you maje a pigs ear of it then you can dissolve the glue by using acetone (as you discovered). For bonding lens surfaces now, say inside a diving mask to give a spectacle correction, a UV setting glue is now used (if you cock that up it's a new pair of lenses).

That brings back memories. When I was a teenager, I used to do a bit of amateur microscopy and prepare permanent slides with canada balsam. My efforts were really crappy - I never had a good practical aptitude for fine detail, but I enjoyed it. Used to embed stuff in wax and section it. I used to use xylene for a canada balsam solvent, didn't realize it was carcinogenic !

Edit: yes, I know, I should have been chasing girls......

The_Metatron, you're a genius, but I'll bet your acetone and mineral oil won't work on plastic lenses.

I was thinking of modifying my glasses, uh, plastic eyeware, but I couldn't think of any chemicals to use that wouldn't destroy the lenses.

Grace wrote:The_Metatron, you're a genius, but I'll bet your acetone and mineral oil won't work on plastic lenses.

I was thinking of modifying my glasses, uh, plastic eyeware, but I couldn't think of any chemicals to use that wouldn't destroy the lenses.

Depends on what you intend to use and if there's any coating present, but plastic corrective lenses can take acetone, alcohol, white spirits, and most other household solvents. If there's anti-reflective coating, there are substances that can destroy it, but acetone doesn't affect it.

A friend of my was using alcohol to clean her plastic lenses. She couldn't figure out why she wasn't able to see well with her glasses on. She took them off and handed them to me. I took a laser scanner to the lenses and she could see the many finite scratches all over both lenses. It looked like she took sand paper to it. I told her next time use proper cleaner on the lenses and not alcohol.

Alcohol doesn't scratch lenses. Using paper towels, a shirt hem, an unwashed cleaning cloth, etc, will result in fine scratches, especially if the lenses are made from polycarbonate (which you Americans seem fond of; it is tough, but scratches easily). I have worked with eyeglass lenses since 1981, and I've never seen them damaged from solvents.

There are several reasons why plastic is preferable. It weighs about half of what mineral glass weighs, which is important when greater powers and larger frames are involved. It has better/far better UV absorption. It doesn't crack or chip as easily (while incidents where cracked lenses have damaged eyes are rare, it is a factor which has made most workplaces ban mineral lenses). In higher refractive indexes, mineral glass is relatively soft, and with modern coatings on plastic lenses, there's no difference when it comes to scratch resistance. I've been wearing glasses since I was 14, and apart from one pair, it has been plastic all the way. I have had very few scratches, and that's even when not following the care tips I tell my customers...

Grace wrote:A friend of my was using alcohol to clean her plastic lenses. She couldn't figure out why she wasn't able to see well with her glasses on. She took them off and handed them to me. I took a laser scanner to the lenses and she could see the many finite scratches all over both lenses. It looked like she took sand paper to it. I told her next time use proper cleaner on the lenses and not alcohol.

Her lenses were made out of polycarbonate. This is quite a popular material in the States, principally because it is easy to work. In Europe it is marketed by Essilaor as Airwear. While most plastic materials will resist acetone, if the lenses are polycarbonate then it is a big No-No.

Yous have to be careful when using acetone on a plastic (cellulose acetate) spectacle frame as it is often used as a fixative for non load bearing parts. Also in the past when having had to repair a frame you could use acetone to help polish the frame, by removing the file marks.

The_Metatron wrote:I prefer glass lenses. Far more resistant to scratching.

Pisses me off they won't make my little boy's glasses with glass lenses. They tell me bullshit about safety concerns. Goddammit, polycarbonate lenses didn't exist when I was a kid, and I never once broke a lens. So, he has these thick plastic things, and they get scratched to hell.

We would use either laminated or toughened lenses for the more boisterous children, back in the dark ages. Nowadays CR39 with a hard coat or from the optical point of view Trivex or a 1.6 index plastic. Optically polycarbonate is a crap material its Abbe number is the lowest of any of the optical materials used for spectacle lenses, the chromatic abberation is horrendous.

Optical crown glass is, optically, still one of the best matrials going, however from the impact resistance point of view, the specific gravity most of the plastics are better. When I first started some 30 years ago, in the UK, it was 90% glass 10% plastic it is now the other way round. A prediction I have made over the last couple of years is that glass spectacle lenses will not exist in the UK in the next 5 to 10 years, except for a couple of very special purposes eg protective prescription lenses for X-ray technicians.

One of the reasons for the large use of plastic is the cheapness of manufacture and also the ablity to work complicated surfaces on the lens compared to glass. The developments in the pipeline include a machine with the plastic monomer coming in at one end, a round clear dics of meterial being cst, and then both surfaces of the lens being worked by high end CNC lathes, with a finished lens coming out at the other end ready to be put into a spectacle frame with the whole process being untouched by human hand. And it would handle single, vision, bifocal lenses, or progressive lenses in the order they are entered into the system.