Google previews web pages in results

Google are offering previews of web pages in your results. People have been reporting this feature for many days now but today is the first time I've seen it in action. Run your search as normal and next to each result in your list you should see a magnifying glass. Click on it and Google gives you a preview of the page with the areas containing your text outlined and the text in boxes superimposed on top of the preview. Although it is a useful tool for assessing the relevance of a page I do find it irritating that Google then automatically previews every result as I move my mouse around the results page. You have to click on the magnifying glass again to switch it off.