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Hi Steve, Feb 14, 2014
Sorry for the delayed reply. I waited until I had time to find the
article on line so others who might be interested but did not have NG at
hand could see the images. Good luck with that. Now one must jump through
flaming hoops and undergo ordeal by water to get in.
Also I am now convinced that the convention I referred to is the
exception rather than the rule.
When I view a mm ruler through my 14X hand lens with one eye and the
ruler with the other eye, the image in the lens is equivalent to about 2.5
mm; consistent with magnification expressed as area and this appears to be
the exception.
When I do the same thing with a dissecting microscope at 20X and 40X, I
get severe eye strain when looking through the mike with one eye and viewing
a ruler with the other, but one mm does appear to be about 20 and 40 mm long
respectively; magnification expressed on a linear basis.
I understand that if the leading number in binocular specs. is 7 then an
object viewed 70' away appears to be 10' away; magnification expressed on a
linear basis.
I calibrated my ocular micrometer in my antique compound mike years ago,
when I had access to a calibrated slide, and I now notice that the length of
one scale division with a 10X objective is about 4 times as large as is a
division is when viewed with a 40X objective; consistent with linear
magnification.
Going back to the NG article, the first image is a section 1000 microns
tall and a pale area about 1/10 as tall is projected as image 2, 100 microns
tall and described as a "section a hundredth the size". In going from image
2 to image 3, a pale area about 1/10 as tall is projected to #3 and is 10
microns tall as expected but is described as "Magnified again by 100."
Note that each of these two steps, both 10x linear magnification, are
described as being 100 fold magnification.
As you observe, the only safe way to express magnification is a scale
line with length stated. Many, myself included, learned this the hard way.
This article does explain one thing clearly. The structure looks very
disorderly and almost random so I now understand why I frequently don't know
why I went down to the basement.until I am half-way back up.
Yt, Dave Webster, Kentville
----- Original Message -----
From: "Stephen Shaw" <srshaw@Dal.Ca>
To: <naturens@chebucto.ns.ca>
Sent: Saturday, February 01, 2014 1:17 AM
Subject: RE: [NatureNS] magnification/reduction
> Hi Dave,
> I usually understand your posts but this one is a puzzle, so I'm sure I'm
> missing something.
> There are two conventions for expressing magnification of a brain slice
> or section on the published page, originally made down a microscope or
> with other some kind of imaging system. Both involve the linear dimension
> (mm, µm, nm), not area. The first convention involves drawing a short
> line on the Figure that represents the original length of that line in the
> tissue, and next to it writing '1 mm', '10 µm' or whatever that length was
> (or it gets written in the figure legend as 'scale bar : 10µm'). In
> optical microscopes, this is based on photographing a calibrated
> mini-ruler at the same microscope magnification. The second convention
> that used to be required by some publications involved stating in the
> figure legend something like 'magnification: 40x', meaning that the linear
> dimension had been scaled up 40 times, so that 100 µm on the original
> section was now 4000 µm (4 mm) on the submitted figure, which usually used
> to be a photographic print. This was/is a dangerous method, because if
> the printer decided to reduce the size by (say) half to publish it, as
> often happened, the mag factor was then wrong (then should be 20x instead
> of 40x). In the field, telescopes and binocs are classified this way as
> 10x or whatever, but this represents the magnification at the eye, not in
> a subsequent image, which would still have to be calibrated. Nowadays
> it's mostly digital images from microscopes so only convention 1 is safe.
>
> You can of course make subsidiary statements about area, a usefully
> imaginative one from undergraduate textbooks being that the human
> neocortex when fully flattened out is 'about the area of a large pizza'
> (linear dimension usually not given).
>
> The now very common pictures of brain scans expressed in 2-D with false
> colour squares in them are usually calibrated in voxels (3-D cubic
> elements, basically mm^3), but these appear as squares on the page because
> the page is 2-D. These are very low resolution images (usually limited to
> 2 mm best resolution of the imager, often worsened by the need to average
> several voxels to reduce noise). This is a factor of ~10,000-100,000
> worse than would be needed if you wanted to visualize by such methods
> parts of the individual nerve cells that do the heavy lifting in the
> brain.
> Steve (Hfx)
> ________________________________________
> From: naturens-owner@chebucto.ns.ca [naturens-owner@chebucto.ns.ca] on
> behalf of David & Alison Webster [dwebster@glinx.com]
> Sent: Friday, January 31, 2014 8:28 PM
> To: NatureNS@chebucto.ns.ca
> Subject: [NatureNS] magnification/reduction
>
> Dear All, Jan 31, 2014
> The Feb. issue of National Geographic has an article on the brain which
> nicely illustrates a common convention for expressing magnification in
> terms
> of area as opposed to length (foldout after page 43; magnification by 100
> decreases length to 1/10).
> But when something is represented at say 1/3 natural size the linear
> dimensions are decreased to 1/3 of natural.
>
> Does anyone know the background of the convention for expressing
> magnification in terms of area ? Does this convention apply only to
> objects
> viewed through some optical instrument; mike, telescope, binoc.?
>
> Is it perhaps because, for equal image brightness, light
> intensity/capture/sensitivity must be increased to the same extent as does
> magnification expressed on an area basis ?
>
> Yt, Dave Webster, Kentville
>
>
>
> .
>
>
> -----
> No virus found in this message.
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>
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