[Microbiology] Re: worm count

Larry Farrell via microbio%40net.bio.net (by farrlarr from isu.edu)
Sun Feb 27 11:22:48 EST 2011

On 2/26/2011 10:03 PM, Joy Young wrote:
> I  live on  a farm and looking to buy a microscope to do worm count on my animals, I wondered if a student microscope would be satisfactory I am told that I require a magnification of 100 x 400.  I am not familiar with microscopes but would be pleased to learn.
> Joy Young

You will not need a magnification of 100 x 400 (a total magnification of 
40,000X); studying bacteria only requires a 10 x 100 magnification(a 
total of 1,000X), and there is no reason to go much beyond that total 
magnification because the optical properties of light make anything much 
beyond that level what is called "empty magnification."  That is, 
although the scope is theoretically capable of magnifying beyond that 
level, it doesn't give you any more information about the subject than 
what you can see at 1,000X.  A student microscope will work fine, 
depending on what kind you get.  If you look at toy store catalogs and 
get what they call a student microscope, it probably will not be worth 
your money/time.  Instead, I suggest that you check out Edmund 
Scientific online and plan to spend at least $150-200 on one of their 
scopes.  If you can afford more than that, even better; the more money 
you put into the scope, the better optics you can expect.

Realistically, "worm counts" will probably not require anything beyond 
about 400X (10X eyepiece times 40X objective).  If you go much beyond 
that level of objective, you will need to use oil to observe your 
specimen (oil objective lenses are sufficiently high magnification that 
too little light can be captured directly to allow you to see; the oil 
prevents the light from bending outward, away from the lens, and results 
in more illumination) and that is messy and unnecessary, given what you 
will be looking at.

If there is a college or university in your area, I strongly suggest 
that you (1) consult with some of the biology faculty about your needs, 
discussing specifically with them what type of scope to get, and (2) see 
if they have any surplus scopes you might buy (that would assure a 
reasonable level of quality and function; if the scopes have held up 
under student use, they should work well for you).

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