Why is the blue line in the Balmer series sometimes not visible? So I've conducted an experiment to find the four visible hydrogen emission spectrum lines in the Balmer series in a laboratory. I don't have any background in quantum physics.
When I looked through the eyepiece, I saw the red light, the pale blue light, and the purple light as shown in the picture below:
 
I've asked my lab instructor why couldn't I see the blue light and he said it is well-known that sometimes not all the spectrum is shown. He told me to look it up on the internet, since I didn't take the quantum physics course yet.
Can someone explain to me this phenomenon? Or at least refer me to an article which discusses this issue? The experiment revolved around Balmer series only. (It was my first spectroscopy experiment)
The setup looked like this: 
It looked like this:

Edit:
These are the wavelengths I've found:

So I think the missing wavelength is actually the $410 nm$.
What can be the reason for this?
 A: I was looking through some NIST atomic data for the Balmer Series from here: http://www.nist.gov/srd/upload/jpcrd382009565p.pdf
It lists the spontaneous emission rates for the Balmer series as follows:


*

*$\lambda = 656 \text{ nm}$, $A_{32} = 4.41\text{e+}7\text{ s}^{-1}$

*$\lambda = 486 \text{ nm}$, $A_{42} = 8.42\text{e+}6\text{ s}^{-1}$

*$\lambda = 434 \text{ nm}$, $A_{52} = 2.53\text{e+}6\text{ s}^{-1}$

*$\lambda = 410 \text{ nm}$, $A_{62} = 9.73\text{e+}5\text{ s}^{-1}$

*$\lambda = 397 \text{ nm}$, $A_{72} = 4.38\text{e+}5\text{ s}^{-1}$


So, the lines should certainly get dimmer as you move toward the UV, but I don't see any reason from a quantum mechanics perspective that the blue line at $\lambda = 434 \text{ nm}$ should be dimmer than the line at $\lambda = 410\text{ nm}$
Based you edit above, you seem to be missing the 410 line.  It might not be seen because 1. the spontaneous emission rate is lowered and 2. your eyes are not as sensitive in this part of the EM spectrum. 3. As dmckee pointed out in your comments section, being a higher excited state, it will also be less populated that lower lying levels, meaning the total spontaneous emission from this state will be even further decreased.
A: 
Why is the blue line in the Balmer series sometimes not visible?

The human eye has difficulties in distinguishing dark blue lines on a black background.
You can use "Microsoft Word" to draw a black rectangle and a few dark blue lines of different thicknesses on the rectangle. The thinner the dark blue line the less visible it is. Even the thick dark blue lines are not quite visible.
