the .index() with No Arguments ?

I test the code with jquery 1.10.1 in firefox. and the result is opposite with the 
discription 'When .index() is called on a jQuery object that contains more than one element, it does not calculate the index of the first element as might be expected, but instead calculates the index of the last element. ' .  
![tt](https://f.cloud.github.com/assets/4495830/773815/d355ef44-e943-11e2-8753-c5e03ca61fd3.png)