Thursday, 20 February 2020

Optical Instruments and Their Makers

It’s been a while since I’ve updated you on my work in progress, so here goes: It’s non-fiction, and the title is Brass & Glass: Optical Instruments and Their Makers.

The inspiration for the book came from my interests in stargazing, photography, microscopy and bird-watching. These involve using telescopes, microscopes, binoculars and cameras, and soon I began to take an interest in classic and vintage optical instruments. I found the lack of reference material frustrating - I would see an interesting looking vintage telescope at auction, but to find out more about the maker I would have to buy extraordinarily expensive reference books, many of which are out of print. Further, the information I sought was frequently scattered around multiple sources, and time-consuming to collate. I wanted an affordable, simple, encyclopaedic reference in which I could find the name of the maker or optical instrument and a concise, but informative article about them. This book is my attempt to remedy that gap in the literature.

It’s up to over 1700 entries now, and I’m working with several subject-matter experts to get critique feedback on the content. That will lead to a new raft of updates and additions then, when that’s complete, hopefully it’ll be close to being ready for publication.

Watch this space...

Thursday, 13 February 2020

Using Crossed Polarising Filters on a Conventional Microscope

A short while ago I posted an article about sugar crystals imaged in a microscope with crossed polarising filters. I promised to show the setup I used to achieve this, so here we are.

What you see here is a Charles Baker microscope with a Cooke microscope lamp providing Köhler Illumination. (The link is to my previous article explaining Köhler Illumination). I've implemented the crossed polarisation very simply.

I have a small polarising sheet that I have cut into two two-inch squares.
I use one of the squares in a filter slot in the microscope light.
And I place the other over the microscope slide, but under the objective lens. This sheet of polarising filter I rotate, while viewing through the microscope, until I see the best contrast.
And the result... I get to see these wonderfully colourful renditions of the crystalline structure of the sugar (see my earlier post).

A microscope used with crossed polarising filters is sometimes referred to as a polariscope.


In general, a polariscope is an optical inspection device used to detect internal stresses in glass and other transparent materials such as plastics, synthetic resins, crystalline materials, etc. A polariscope is composed chiefly of a light source and two crossed polarised lenses. Material to be examined is placed between the two polariscope lenses and viewed through the lens opposite the light source lens. It is commonly used in detecting the optical properties of gemstones.

All content and images © Tony Benson

Monday, 3 February 2020

Köhler Illumination

Köhler Illumination is a form of microscope sample illumination, invented by August Köhler, (see below), that provides even light across the field of view, and a light cone that matches the numerical aperture of the objective lens. It is suitable for either viewing or photography. It normally consists of a collector lens near the light source, together with a field iris, and a sub-stage condenser with a condenser iris. The focus of both the collector and condenser lenses is adjustable, as is the aperture of each of the irises.

The Cooke microscope light you see here has both an adjustable collector lens (the big lever pointing straight up), and an adjustable field iris (the small lever you see further to the right). You can click on the image to see it enlarged.

It also has a knob to adjust how bright the light is, and slots where filters can be inserted. This particular model of light is designed by Cooke to be used with a Cooke microscope, but it can easily be used with any conventional microscope.

The microscope I've shown in the picture above is by Charles Baker. It has a sub-stage condenser and iris, and a sub-stage mirror. The mirror has a plane face and a curved face. The plane face of the mirror is used in conjunction with Köhler Illumination, since the focusing of the illuminating light is achieved by the collector lens and the sub-stage condenser.

Using Köhler Illumination

The collector lens is focused to form an image of the light source in the plane of the sub-stage condenser iris. The condenser lens is focused to form an image of the field iris in the sample plane. The two irises are adjusted to provide the required light cone. Thus, when the sample is in focus the image of the light source is completely de-focused, resulting in even illumination.

Some microscopes have built-in Köhler illumination. In this case, the principle is the same, but the practicalities of the adjustments may be different.

August Köhler

August Köhler, (1866-1948), was a German scientist who made several innovations in microscopy. He is best known for his invention of Köhler Illumination in 1893. In 1900 he began to work for Carl Zeiss, where he continued to work until 1945, three years before his death.

All content and images © Tony Benson